From 599f8462094661da909d535da81d9fb7fe90ea96 Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Fri, 30 May 2025 14:45:33 +0200
Subject: [PATCH 01/28] Fix SSR non-streaming: rendering has higher priority
 than re-execution.

---
 .../Endpoints/src/RazorComponentEndpointInvoker.cs         | 7 -------
 .../src/Rendering/EndpointHtmlRenderer.EventDispatch.cs    | 1 -
 2 files changed, 8 deletions(-)

diff --git a/src/Components/Endpoints/src/RazorComponentEndpointInvoker.cs b/src/Components/Endpoints/src/RazorComponentEndpointInvoker.cs
index 245f811d7f76..39aefaf164e2 100644
--- a/src/Components/Endpoints/src/RazorComponentEndpointInvoker.cs
+++ b/src/Components/Endpoints/src/RazorComponentEndpointInvoker.cs
@@ -102,13 +102,6 @@ await _renderer.InitializeStandardComponentServicesAsync(
             ParameterView.Empty,
             waitForQuiescence: result.IsPost || isErrorHandlerOrReExecuted);
 
-        bool avoidStartingResponse = hasStatusCodePage && !isReExecuted && context.Response.StatusCode == StatusCodes.Status404NotFound;
-        if (avoidStartingResponse)
-        {
-            // the request is going to be re-executed, we should avoid writing to the response
-            return;
-        }
-
         Task quiesceTask;
         if (!result.IsPost)
         {
diff --git a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
index 6af020fc847a..3ae2f6fcd36a 100644
--- a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
+++ b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
@@ -91,7 +91,6 @@ private async Task SetNotFoundResponseAsync(string baseUri)
         else
         {
             _httpContext.Response.StatusCode = StatusCodes.Status404NotFound;
-            _httpContext.Response.ContentType = null;
         }
 
         // When the application triggers a NotFound event, we continue rendering the current batch.

From 842941efd3fd8f015109e997cefd0f313a3a66c9 Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Fri, 30 May 2025 14:48:35 +0200
Subject: [PATCH 02/28] Add tests: move component common for global and local
 interactivity to shared project + redefine test cases.

---
 .../ServerRenderingTests/InteractivityTest.cs | 59 +++++++++++++++++++
 .../NoInteractivityTest.cs                    | 54 ++++++++---------
 .../StatusCodePagesTest.cs                    | 39 ------------
 .../ComponentThatSetsNotFound.razor           | 26 ++++++++
 .../Components.Shared.csproj                  | 17 ++++++
 .../NotFoundPage.razor                        |  0
 .../PageThatSetsNotFound-no-streaming.razor   | 11 ++++
 .../PageThatSetsNotFound-streaming.razor      | 11 ++++
 .../ReexecutedPage.razor                      |  0
 .../Components.Shared/_Imports.razor          |  1 +
 .../Components.TestServer.csproj              |  1 +
 ...omponentEndpointsNoInteractivityStartup.cs | 15 ++++-
 .../RazorComponentEndpointsStartup.cs         |  9 +++
 .../RazorComponents/App.razor                 |  2 +-
 .../Pages/NotFound/PageThatSetsNotFound.razor | 23 --------
 .../StreamingSetNotFound.razor                | 30 ----------
 .../PageThatSetsNotFound-Interactive.razor    | 28 +++++++++
 .../Components.WasmMinimal/Routes.razor       |  2 +-
 18 files changed, 201 insertions(+), 127 deletions(-)
 delete mode 100644 src/Components/test/E2ETest/ServerRenderingTests/StatusCodePagesTest.cs
 create mode 100644 src/Components/test/testassets/Components.Shared/ComponentThatSetsNotFound.razor
 create mode 100644 src/Components/test/testassets/Components.Shared/Components.Shared.csproj
 rename src/Components/test/testassets/{Components.TestServer/RazorComponents/Pages/NotFound => Components.Shared}/NotFoundPage.razor (100%)
 create mode 100644 src/Components/test/testassets/Components.Shared/PageThatSetsNotFound-no-streaming.razor
 create mode 100644 src/Components/test/testassets/Components.Shared/PageThatSetsNotFound-streaming.razor
 rename src/Components/test/testassets/{Components.TestServer/RazorComponents/Pages/NotFound => Components.Shared}/ReexecutedPage.razor (100%)
 create mode 100644 src/Components/test/testassets/Components.Shared/_Imports.razor
 delete mode 100644 src/Components/test/testassets/Components.TestServer/RazorComponents/Pages/NotFound/PageThatSetsNotFound.razor
 delete mode 100644 src/Components/test/testassets/Components.TestServer/RazorComponents/Pages/StreamingRendering/StreamingSetNotFound.razor
 create mode 100644 src/Components/test/testassets/Components.WasmMinimal/Pages/NotFound/PageThatSetsNotFound-Interactive.razor

diff --git a/src/Components/test/E2ETest/ServerRenderingTests/InteractivityTest.cs b/src/Components/test/E2ETest/ServerRenderingTests/InteractivityTest.cs
index dd9f2a3a47e8..ecac592aac33 100644
--- a/src/Components/test/E2ETest/ServerRenderingTests/InteractivityTest.cs
+++ b/src/Components/test/E2ETest/ServerRenderingTests/InteractivityTest.cs
@@ -1448,4 +1448,63 @@ public void BrowserNavigationToNotExistingPathReExecutesTo404(string renderMode)
 
     private void Assert404ReExecuted() =>
         Browser.Equal("Welcome On Page Re-executed After Not Found Event", () => Browser.Exists(By.Id("test-info")).Text);
+
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public void CanRenderNotFoundPage_SSR(bool streamingStarted)
+    {
+        string streamingPath = streamingStarted ? "-streaming" : "";
+        Navigate($"{ServerPathBase}/set-not-found-ssr{streamingPath}?useCustomNotFoundPage=true");
+        AssertCustomNotFoundPageRendered()
+    }
+
+    [Theory]
+    [InlineData("ServerNonPrerendered")]
+    [InlineData("WebAssemblyNonPrerendered")]
+    public void CanRenderNotFoundPage_Interactive(string renderMode)
+    {
+        string streamingPath = streamingStarted ? "-streaming" : "";
+        Navigate($"{ServerPathBase}/set-not-found?useCustomNotFoundPage=true&renderMode={renderMode}");
+        AssertCustomNotFoundPageRendered();
+    }
+
+    private void AssertCustomNotFoundPageRendered()
+    {
+        var infoText = Browser.FindElement(By.Id("test-info")).Text;
+        Assert.Contains("Welcome On Custom Not Found Page", infoText);
+        // custom page should have a custom layout
+        var aboutLink = Browser.FindElement(By.Id("about-link")).Text;
+        Assert.Contains("About", aboutLink);
+    }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void DoesNotReExecuteIf404WasHandled_SSR(bool streamingStarted)
+    {
+        string streamingPath = streamingStarted ? "-streaming" : "";
+        Navigate($"{ServerPathBase}/reexecution/set-not-found-ssr{streamingPath}");
+        AssertNotFoundFragmentRendered();
+    }
+
+    [Theory]
+    [InlineData("ServerNonPrerendered")]
+    [InlineData("WebAssemblyNonPrerendered")]
+    public async Task DoesNotReExecuteIf404WasHandled_Interactive(string renderMode)
+    {
+        Navigate($"{ServerPathBase}/reexecution/set-not-found?renderMode={renderMode}");
+        await Task.Delay(5000);
+        AssertNotFoundFragmentRendered();
+    }
+
+    private void AssertNotFoundFragmentRendered() =>
+        Browser.Equal("There's nothing here", () => Browser.FindElement(By.CssSelector("body > p")).Text);
+
+    [Fact]
+    public void StatusCodePagesWithReExecution()
+    {
+        Navigate($"{ServerPathBase}/reexecution/trigger-404");
+        Assert404ReExecuted();
+    }
 }
diff --git a/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs b/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
index 35f7140f9170..26399b8f556f 100644
--- a/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
+++ b/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
@@ -80,13 +80,6 @@ public void NavigatesWithoutInteractivityByRequestRedirection(bool controlFlowBy
         Browser.Equal("Routing test cases", () => Browser.Exists(By.Id("test-info")).Text);
     }
 
-    [Fact]
-    public void CanRenderNotFoundPageAfterStreamingStarted()
-    {
-        Navigate($"{ServerPathBase}/streaming-set-not-found");
-        Browser.Equal("Default Not Found Page", () => Browser.Title);
-    }
-
     [Theory]
     [InlineData(true)]
     [InlineData(false)]
@@ -127,36 +120,35 @@ private void Assert404ReExecuted() =>
     [Theory]
     [InlineData(true)]
     [InlineData(false)]
-    public void CanRenderNotFoundPageNoStreaming(bool useCustomNotFoundPage)
+    public void CanRenderNotFoundPage(bool streamingStarted)
     {
-        string query = useCustomNotFoundPage ? "&useCustomNotFoundPage=true" : "";
-        Navigate($"{ServerPathBase}/set-not-found?shouldSet=true{query}");
-
-        if (useCustomNotFoundPage)
-        {
-            var infoText = Browser.FindElement(By.Id("test-info")).Text;
-            Assert.Contains("Welcome On Custom Not Found Page", infoText);
-            // custom page should have a custom layout
-            var aboutLink = Browser.FindElement(By.Id("about-link")).Text;
-            Assert.Contains("About", aboutLink);
-        }
-        else
-        {
-            var bodyText = Browser.FindElement(By.TagName("body")).Text;
-            Assert.Contains("There's nothing here", bodyText);
-        }
+        string streamingPath = streamingStarted ? "-streaming" : "";
+        Navigate($"{ServerPathBase}/set-not-found-ssr{streamingPath}?useCustomNotFoundPage=true");
+
+        var infoText = Browser.FindElement(By.Id("test-info")).Text;
+        Assert.Contains("Welcome On Custom Not Found Page", infoText);
+        // custom page should have a custom layout
+        var aboutLink = Browser.FindElement(By.Id("about-link")).Text;
+        Assert.Contains("About", aboutLink);
     }
 
     [Theory]
-    [InlineData(true)]
     [InlineData(false)]
-    public void CanRenderNotFoundPageWithStreaming(bool useCustomNotFoundPage)
+    [InlineData(true)]
+    public void DoesNotReExecuteIf404WasHandled(bool streamingStarted)
     {
-        // when streaming started, we always render page under "not-found" path
-        string query = useCustomNotFoundPage ? "?useCustomNotFoundPage=true" : "";
-        Navigate($"{ServerPathBase}/streaming-set-not-found{query}");
+        string streamingPath = streamingStarted ? "-streaming" : "";
+        Navigate($"{ServerPathBase}/reexecution/set-not-found-ssr{streamingPath}");
+        AssertNotFoundFragmentRendered();
+    }
+
+    private void AssertNotFoundFragmentRendered() =>
+        Browser.Equal("There's nothing here", () => Browser.FindElement(By.CssSelector("body > p")).Text);
 
-        string expectedTitle = "Default Not Found Page";
-        Browser.Equal(expectedTitle, () => Browser.Title);
+    [Fact]
+    public void StatusCodePagesWithReExecution()
+    {
+        Navigate($"{ServerPathBase}/reexecution/trigger-404");
+        Browser.Equal("Re-executed page", () => Browser.Title);
     }
 }
diff --git a/src/Components/test/E2ETest/ServerRenderingTests/StatusCodePagesTest.cs b/src/Components/test/E2ETest/ServerRenderingTests/StatusCodePagesTest.cs
deleted file mode 100644
index 58ac90b39bbe..000000000000
--- a/src/Components/test/E2ETest/ServerRenderingTests/StatusCodePagesTest.cs
+++ /dev/null
@@ -1,39 +0,0 @@
-// Licensed to the .NET Foundation under one or more agreements.
-// The .NET Foundation licenses this file to you under the MIT license.
-
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
-using Components.TestServer.RazorComponents;
-using Components.TestServer.RazorComponents.Pages.StreamingRendering;
-using Microsoft.AspNetCore.Components.E2ETest.Infrastructure;
-using Microsoft.AspNetCore.Components.E2ETest.Infrastructure.ServerFixtures;
-using Microsoft.AspNetCore.E2ETesting;
-using OpenQA.Selenium;
-using TestServer;
-using Xunit.Abstractions;
-
-namespace Microsoft.AspNetCore.Components.E2ETests.ServerRenderingTests;
-
-public class StatusCodePagesTest(BrowserFixture browserFixture, BasicTestAppServerSiteFixture<RazorComponentEndpointsStartup<App>> serverFixture, ITestOutputHelper output)
-    : ServerTestBase<BasicTestAppServerSiteFixture<RazorComponentEndpointsStartup<App>>>(browserFixture, serverFixture, output)
-{
-
-    [Theory]
-    [InlineData(false, false)]
-    [InlineData(true, false)]
-    [InlineData(true, true)]
-    public void StatusCodePagesWithReExecution(bool streaming, bool responseStarted)
-    {
-        string streamingPath = streaming ? "streaming-" : "";
-        Navigate($"{ServerPathBase}/reexecution/{streamingPath}set-not-found?responseStarted={responseStarted}");
-
-        // streaming when response started does not re-execute
-        string expectedTitle = responseStarted
-            ? "Default Not Found Page"
-            : "Re-executed page";
-        Browser.Equal(expectedTitle, () => Browser.Title);
-    }
-}
diff --git a/src/Components/test/testassets/Components.Shared/ComponentThatSetsNotFound.razor b/src/Components/test/testassets/Components.Shared/ComponentThatSetsNotFound.razor
new file mode 100644
index 000000000000..fe024d3bb8d0
--- /dev/null
+++ b/src/Components/test/testassets/Components.Shared/ComponentThatSetsNotFound.razor
@@ -0,0 +1,26 @@
+@inject NavigationManager NavigationManager
+
+@if (!WaitForInteractivity || RendererInfo.IsInteractive)
+{
+    <PageTitle>Original page</PageTitle>
+
+    <p id="test-info">Any content</p>
+
+}
+
+@code{
+    [Parameter]
+    public bool StartStreaming { get; set; } = false;
+
+    [Parameter]
+    public bool WaitForInteractivity { get; set; } = false;
+
+    protected async override Task OnInitializedAsync()
+    {
+        if (StartStreaming)
+        {
+            await Task.Yield();
+        }
+        NavigationManager.NotFound();
+    }
+}
\ No newline at end of file
diff --git a/src/Components/test/testassets/Components.Shared/Components.Shared.csproj b/src/Components/test/testassets/Components.Shared/Components.Shared.csproj
new file mode 100644
index 000000000000..e2b71dba2459
--- /dev/null
+++ b/src/Components/test/testassets/Components.Shared/Components.Shared.csproj
@@ -0,0 +1,17 @@
+<Project Sdk="Microsoft.NET.Sdk.Razor">
+
+  <PropertyGroup>
+    <TargetFramework>$(DefaultNetCoreTargetFramework)</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <SupportedPlatform Include="browser" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Reference Include="Microsoft.AspNetCore.Components.Web" />
+  </ItemGroup>
+
+</Project>
\ No newline at end of file
diff --git a/src/Components/test/testassets/Components.TestServer/RazorComponents/Pages/NotFound/NotFoundPage.razor b/src/Components/test/testassets/Components.Shared/NotFoundPage.razor
similarity index 100%
rename from src/Components/test/testassets/Components.TestServer/RazorComponents/Pages/NotFound/NotFoundPage.razor
rename to src/Components/test/testassets/Components.Shared/NotFoundPage.razor
diff --git a/src/Components/test/testassets/Components.Shared/PageThatSetsNotFound-no-streaming.razor b/src/Components/test/testassets/Components.Shared/PageThatSetsNotFound-no-streaming.razor
new file mode 100644
index 000000000000..6ac4db557e13
--- /dev/null
+++ b/src/Components/test/testassets/Components.Shared/PageThatSetsNotFound-no-streaming.razor
@@ -0,0 +1,11 @@
+@page "/reexecution/set-not-found-ssr"
+@page "/set-not-found-ssr"
+@attribute [StreamRendering(false)]
+
+@*
+    this page is used in global interactivity and no interactivity scenarios
+    the content is rendered on the server without streaming and might become
+    interactive later if interactivity was enabled in the app
+*@
+
+<Components.Shared.ComponentThatSetsNotFound />
\ No newline at end of file
diff --git a/src/Components/test/testassets/Components.Shared/PageThatSetsNotFound-streaming.razor b/src/Components/test/testassets/Components.Shared/PageThatSetsNotFound-streaming.razor
new file mode 100644
index 000000000000..061d35386bc0
--- /dev/null
+++ b/src/Components/test/testassets/Components.Shared/PageThatSetsNotFound-streaming.razor
@@ -0,0 +1,11 @@
+@page "/reexecution/set-not-found-ssr-streaming"
+@page "/set-not-found-ssr-streaming"
+@attribute [StreamRendering(true)]
+
+@*
+    this page is used in global interactivity and no interactivity scenarios
+    the content is rendered on the server with streaming and might become
+    interactive later if interactivity was enabled in the app
+*@
+
+<Components.Shared.ComponentThatSetsNotFound StartStreaming="true"/>
\ No newline at end of file
diff --git a/src/Components/test/testassets/Components.TestServer/RazorComponents/Pages/NotFound/ReexecutedPage.razor b/src/Components/test/testassets/Components.Shared/ReexecutedPage.razor
similarity index 100%
rename from src/Components/test/testassets/Components.TestServer/RazorComponents/Pages/NotFound/ReexecutedPage.razor
rename to src/Components/test/testassets/Components.Shared/ReexecutedPage.razor
diff --git a/src/Components/test/testassets/Components.Shared/_Imports.razor b/src/Components/test/testassets/Components.Shared/_Imports.razor
new file mode 100644
index 000000000000..2b9557ce5a53
--- /dev/null
+++ b/src/Components/test/testassets/Components.Shared/_Imports.razor
@@ -0,0 +1 @@
+@using Microsoft.AspNetCore.Components.Web
\ No newline at end of file
diff --git a/src/Components/test/testassets/Components.TestServer/Components.TestServer.csproj b/src/Components/test/testassets/Components.TestServer/Components.TestServer.csproj
index 1341319c942c..1e80ca221765 100644
--- a/src/Components/test/testassets/Components.TestServer/Components.TestServer.csproj
+++ b/src/Components/test/testassets/Components.TestServer/Components.TestServer.csproj
@@ -35,6 +35,7 @@
     <ProjectReference Include="..\BasicTestApp\BasicTestApp.csproj" />
     <ProjectReference Include="..\Components.WasmMinimal\Components.WasmMinimal.csproj" />
     <ProjectReference Include="..\Components.WasmRemoteAuthentication\Components.WasmRemoteAuthentication.csproj" />
+    <ProjectReference Include="..\Components.Shared\Components.Shared.csproj" />
   </ItemGroup>
 
   <ItemGroup>
diff --git a/src/Components/test/testassets/Components.TestServer/RazorComponentEndpointsNoInteractivityStartup.cs b/src/Components/test/testassets/Components.TestServer/RazorComponentEndpointsNoInteractivityStartup.cs
index 4520d9202397..6bc67a18c6fc 100644
--- a/src/Components/test/testassets/Components.TestServer/RazorComponentEndpointsNoInteractivityStartup.cs
+++ b/src/Components/test/testassets/Components.TestServer/RazorComponentEndpointsNoInteractivityStartup.cs
@@ -50,6 +50,15 @@ public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
         {
             app.Map("/reexecution", reexecutionApp =>
             {
+                app.Map("/trigger-404", trigger404App =>
+                {
+                    trigger404App.Run(async context =>
+                    {
+                        context.Response.StatusCode = 404;
+                        await context.Response.WriteAsync("Triggered a 404 status code.");
+                    });
+                });
+
                 if (!env.IsDevelopment())
                 {
                     app.UseExceptionHandler("/Error", createScopeForErrors: true);
@@ -62,7 +71,8 @@ public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
                 reexecutionApp.UseAntiforgery();
                 reexecutionApp.UseEndpoints(endpoints =>
                 {
-                    endpoints.MapRazorComponents<TRootComponent>();
+                    endpoints.MapRazorComponents<TRootComponent>()
+                        .AddAdditionalAssemblies(Assembly.Load("Components.Shared"));
                 });
             });
 
@@ -83,7 +93,8 @@ private void ConfigureSubdirPipeline(IApplicationBuilder app, IWebHostEnvironmen
         app.UseAntiforgery();
         app.UseEndpoints(endpoints =>
         {
-            endpoints.MapRazorComponents<TRootComponent>();
+            endpoints.MapRazorComponents<TRootComponent>()
+                .AddAdditionalAssemblies(Assembly.Load("Components.Shared"));
         });
     }
 }
diff --git a/src/Components/test/testassets/Components.TestServer/RazorComponentEndpointsStartup.cs b/src/Components/test/testassets/Components.TestServer/RazorComponentEndpointsStartup.cs
index ea4f7f7ad220..0c01031da9a7 100644
--- a/src/Components/test/testassets/Components.TestServer/RazorComponentEndpointsStartup.cs
+++ b/src/Components/test/testassets/Components.TestServer/RazorComponentEndpointsStartup.cs
@@ -75,6 +75,14 @@ public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
         {
             app.Map("/reexecution", reexecutionApp =>
             {
+                app.Map("/trigger-404", app =>
+                {
+                    app.Run(async context =>
+                    {
+                        context.Response.StatusCode = 404;
+                        await context.Response.WriteAsync("Triggered a 404 status code.");
+                    });
+                });
                 reexecutionApp.UseStatusCodePagesWithReExecute("/not-found-reexecute", createScopeForErrors: true);
                 reexecutionApp.UseRouting();
 
@@ -125,6 +133,7 @@ private void ConfigureEndpoints(IApplicationBuilder app, IWebHostEnvironment env
             }
 
             _ = endpoints.MapRazorComponents<TRootComponent>()
+                .AddAdditionalAssemblies(Assembly.Load("Components.Shared"))
                 .AddAdditionalAssemblies(Assembly.Load("Components.WasmMinimal"))
                 .AddInteractiveServerRenderMode(options =>
                 {
diff --git a/src/Components/test/testassets/Components.TestServer/RazorComponents/App.razor b/src/Components/test/testassets/Components.TestServer/RazorComponents/App.razor
index ace8d627d941..a4e8ff2dff98 100644
--- a/src/Components/test/testassets/Components.TestServer/RazorComponents/App.razor
+++ b/src/Components/test/testassets/Components.TestServer/RazorComponents/App.razor
@@ -29,7 +29,7 @@
     <HeadOutlet />
 </head>
 <body>
-    <Router AppAssembly="@typeof(App).Assembly" NotFoundPage="NotFoundPageType">
+    <Router AppAssembly="@typeof(App).Assembly" AdditionalAssemblies="new[] { typeof(Components.Shared.NotFoundPage).Assembly }" NotFoundPage="NotFoundPageType">
         <Found Context="routeData">
             <RouteView RouteData="@routeData" />
             <FocusOnNavigate RouteData="@routeData" Selector="[data-focus-on-navigate]" />
diff --git a/src/Components/test/testassets/Components.TestServer/RazorComponents/Pages/NotFound/PageThatSetsNotFound.razor b/src/Components/test/testassets/Components.TestServer/RazorComponents/Pages/NotFound/PageThatSetsNotFound.razor
deleted file mode 100644
index e397f81672a3..000000000000
--- a/src/Components/test/testassets/Components.TestServer/RazorComponents/Pages/NotFound/PageThatSetsNotFound.razor
+++ /dev/null
@@ -1,23 +0,0 @@
-@page "/reexecution/set-not-found"
-@page "/set-not-found"
-@attribute [StreamRendering(false)]
-@inject NavigationManager NavigationManager
-
-<PageTitle>Original page</PageTitle>
-
-<p id="test-info">Any content</p>
-
-@code{
-    [Parameter]
-    [SupplyParameterFromQuery(Name = "shouldSet")]
-    public bool? ShouldSet { get; set; }
-
-    protected override void OnInitialized()
-    {
-        bool shouldSet = ShouldSet ?? true;
-        if (shouldSet)
-        {
-            NavigationManager.NotFound();
-        }
-    }
-}
diff --git a/src/Components/test/testassets/Components.TestServer/RazorComponents/Pages/StreamingRendering/StreamingSetNotFound.razor b/src/Components/test/testassets/Components.TestServer/RazorComponents/Pages/StreamingRendering/StreamingSetNotFound.razor
deleted file mode 100644
index 92b5f95d7c4b..000000000000
--- a/src/Components/test/testassets/Components.TestServer/RazorComponents/Pages/StreamingRendering/StreamingSetNotFound.razor
+++ /dev/null
@@ -1,30 +0,0 @@
-@page "/reexecution/streaming-set-not-found"
-@page "/streaming-set-not-found"
-@attribute [StreamRendering]
-@inject NavigationManager NavigationManager
-
-@code {
-    [Parameter]
-    [SupplyParameterFromQuery(Name = "shouldSet")]
-    public bool? ShouldSet { get; set; }
-
-    [Parameter]
-    [SupplyParameterFromQuery(Name = "responseStarted")]
-    public bool? ResponseStarted { get; set; }
-
-    protected override async Task OnInitializedAsync()
-    {
-        bool shouldSet = ShouldSet ?? true;
-        bool responseStarted = ResponseStarted ?? true;
-        if (responseStarted)
-        {
-            // Simulate some delay before triggering NotFound to start streaming response
-            await Task.Yield();
-        }
-
-        if (shouldSet)
-        {
-            NavigationManager.NotFound();
-        }
-    }
-}
diff --git a/src/Components/test/testassets/Components.WasmMinimal/Pages/NotFound/PageThatSetsNotFound-Interactive.razor b/src/Components/test/testassets/Components.WasmMinimal/Pages/NotFound/PageThatSetsNotFound-Interactive.razor
new file mode 100644
index 000000000000..46c0c65cecf9
--- /dev/null
+++ b/src/Components/test/testassets/Components.WasmMinimal/Pages/NotFound/PageThatSetsNotFound-Interactive.razor
@@ -0,0 +1,28 @@
+@page "/reexecution/set-not-found"
+@page "/set-not-found"
+
+@*
+    this page is used only in global interactivity scenarios
+    the component's content will be rendered when it becomes interactive
+*@
+
+<Components.Shared.ComponentThatSetsNotFound @rendermode="@RenderModeHelper.GetRenderMode(_renderMode)" WaitForInteractivity="true" />
+
+@code{
+    [Parameter, SupplyParameterFromQuery(Name = "renderMode")]
+    public string? RenderModeStr { get; set; }
+
+    private RenderModeId _renderMode;
+
+    protected override void OnInitialized()
+    {
+        if (!string.IsNullOrEmpty(RenderModeStr))
+        {
+            _renderMode = RenderModeHelper.ParseRenderMode(RenderModeStr);
+        }
+        else
+        {
+            throw new ArgumentException("RenderModeStr cannot be null or empty. Did you mean to redirect to /set-not-found-ssr?", nameof(RenderModeStr));
+        }
+    }
+}
\ No newline at end of file
diff --git a/src/Components/test/testassets/Components.WasmMinimal/Routes.razor b/src/Components/test/testassets/Components.WasmMinimal/Routes.razor
index 92f41cc8b4f1..ca687782bc22 100644
--- a/src/Components/test/testassets/Components.WasmMinimal/Routes.razor
+++ b/src/Components/test/testassets/Components.WasmMinimal/Routes.razor
@@ -22,7 +22,7 @@
     }
 }
 
-<Router AppAssembly="@typeof(Program).Assembly" NotFoundPage="NotFoundPageType">
+<Router AppAssembly="@typeof(Program).Assembly" AdditionalAssemblies="new[] { typeof(Components.Shared.NotFoundPage).Assembly }" NotFoundPage="NotFoundPageType">
     <Found Context="routeData">
         <RouteView RouteData="@routeData" />
         <FocusOnNavigate RouteData="@routeData" Selector="h1" />

From 079b66fcac631142bb70660900988fb57c19eb3f Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Fri, 30 May 2025 15:00:13 +0200
Subject: [PATCH 03/28] Missing build fixes for the previous commit.

---
 .../test/E2ETest/ServerRenderingTests/InteractivityTest.cs     | 3 +--
 .../Components.WasmMinimal/Components.WasmMinimal.csproj       | 1 +
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/Components/test/E2ETest/ServerRenderingTests/InteractivityTest.cs b/src/Components/test/E2ETest/ServerRenderingTests/InteractivityTest.cs
index ecac592aac33..c5bc24012ae4 100644
--- a/src/Components/test/E2ETest/ServerRenderingTests/InteractivityTest.cs
+++ b/src/Components/test/E2ETest/ServerRenderingTests/InteractivityTest.cs
@@ -1456,7 +1456,7 @@ public void CanRenderNotFoundPage_SSR(bool streamingStarted)
     {
         string streamingPath = streamingStarted ? "-streaming" : "";
         Navigate($"{ServerPathBase}/set-not-found-ssr{streamingPath}?useCustomNotFoundPage=true");
-        AssertCustomNotFoundPageRendered()
+        AssertCustomNotFoundPageRendered();
     }
 
     [Theory]
@@ -1464,7 +1464,6 @@ public void CanRenderNotFoundPage_SSR(bool streamingStarted)
     [InlineData("WebAssemblyNonPrerendered")]
     public void CanRenderNotFoundPage_Interactive(string renderMode)
     {
-        string streamingPath = streamingStarted ? "-streaming" : "";
         Navigate($"{ServerPathBase}/set-not-found?useCustomNotFoundPage=true&renderMode={renderMode}");
         AssertCustomNotFoundPageRendered();
     }
diff --git a/src/Components/test/testassets/Components.WasmMinimal/Components.WasmMinimal.csproj b/src/Components/test/testassets/Components.WasmMinimal/Components.WasmMinimal.csproj
index deabcd7fccae..ecacf8c5078d 100644
--- a/src/Components/test/testassets/Components.WasmMinimal/Components.WasmMinimal.csproj
+++ b/src/Components/test/testassets/Components.WasmMinimal/Components.WasmMinimal.csproj
@@ -16,6 +16,7 @@
   <ItemGroup>
     <ProjectReference Include="..\NotReferencedInWasmCodePackage\NotReferencedInWasmCodePackage.csproj" />
     <ProjectReference Include="..\TestContentPackage\TestContentPackage.csproj" />
+    <ProjectReference Include="..\Components.Shared\Components.Shared.csproj" />
   </ItemGroup>
 
 </Project>

From 301efac35cdf105f6604c8064ab271c1062aee1d Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Fri, 30 May 2025 15:01:41 +0200
Subject: [PATCH 04/28] Enable `Router` to stream in the `NotFound` content.

---
 .../Components/src/Routing/Router.cs          |  1 +
 .../EndpointHtmlRenderer.EventDispatch.cs     | 19 +++++++------------
 .../src/Rendering/EndpointHtmlRenderer.cs     |  2 +-
 3 files changed, 9 insertions(+), 13 deletions(-)

diff --git a/src/Components/Components/src/Routing/Router.cs b/src/Components/Components/src/Routing/Router.cs
index 51fb3fc823d2..02d22d5bb8c7 100644
--- a/src/Components/Components/src/Routing/Router.cs
+++ b/src/Components/Components/src/Routing/Router.cs
@@ -19,6 +19,7 @@ namespace Microsoft.AspNetCore.Components.Routing;
 /// <summary>
 /// A component that supplies route data corresponding to the current navigation state.
 /// </summary>
+[StreamRendering]
 public partial class Router : IComponent, IHandleAfterRender, IDisposable
 {
     // Dictionary is intentionally used instead of ReadOnlyDictionary to reduce Blazor size
diff --git a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
index 3ae2f6fcd36a..4f308c7984f7 100644
--- a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
+++ b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
@@ -77,26 +77,21 @@ private Task ReturnErrorResponse(string detailedMessage)
             : Task.CompletedTask;
     }
 
-    private async Task SetNotFoundResponseAsync(string baseUri)
+    private void SetNotFoundResponse(object? sender, EventArgs args)
     {
         if (_httpContext.Response.HasStarted)
         {
-            var defaultBufferSize = 16 * 1024;
-            await using var writer = new HttpResponseStreamWriter(_httpContext.Response.Body, Encoding.UTF8, defaultBufferSize, ArrayPool<byte>.Shared, ArrayPool<char>.Shared);
-            using var bufferWriter = new BufferedTextWriter(writer);
-            var notFoundUri = $"{baseUri}not-found";
-            HandleNavigationAfterResponseStarted(bufferWriter, _httpContext, notFoundUri);
-            await bufferWriter.FlushAsync();
+            // We're expecting the Router to continue streaming the NotFound contents
         }
         else
         {
             _httpContext.Response.StatusCode = StatusCodes.Status404NotFound;
-        }
 
-        // When the application triggers a NotFound event, we continue rendering the current batch.
-        // However, after completing this batch, we do not want to process any further UI updates,
-        // as we are going to return a 404 status and discard the UI updates generated so far.
-        SignalRendererToFinishRendering();
+            // When the application triggers a NotFound event, we continue rendering the current batch.
+            // However, after completing this batch, we do not want to process any further UI updates,
+            // as we are going to return a 404 status and discard the UI updates generated so far.
+            SignalRendererToFinishRendering();
+        }
     }
 
     private async Task OnNavigateTo(string uri)
diff --git a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs
index 33e52fa0ed57..cc491759cb20 100644
--- a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs
+++ b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs
@@ -85,7 +85,7 @@ internal async Task InitializeStandardComponentServicesAsync(
 
         if (navigationManager != null)
         {
-            navigationManager.OnNotFound += async (sender, args) => await SetNotFoundResponseAsync(navigationManager.BaseUri);
+            navigationManager.OnNotFound += SetNotFoundResponse;
         }
 
         var authenticationStateProvider = httpContext.RequestServices.GetService<AuthenticationStateProvider>();

From e7ce5baca483bd1c44c0185c4475026378843e4b Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Fri, 30 May 2025 19:06:28 +0200
Subject: [PATCH 05/28] Remove debugging delay.

---
 .../test/E2ETest/ServerRenderingTests/InteractivityTest.cs     | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/src/Components/test/E2ETest/ServerRenderingTests/InteractivityTest.cs b/src/Components/test/E2ETest/ServerRenderingTests/InteractivityTest.cs
index c5bc24012ae4..9536cbbbcee9 100644
--- a/src/Components/test/E2ETest/ServerRenderingTests/InteractivityTest.cs
+++ b/src/Components/test/E2ETest/ServerRenderingTests/InteractivityTest.cs
@@ -1490,10 +1490,9 @@ public void DoesNotReExecuteIf404WasHandled_SSR(bool streamingStarted)
     [Theory]
     [InlineData("ServerNonPrerendered")]
     [InlineData("WebAssemblyNonPrerendered")]
-    public async Task DoesNotReExecuteIf404WasHandled_Interactive(string renderMode)
+    public void DoesNotReExecuteIf404WasHandled_Interactive(string renderMode)
     {
         Navigate($"{ServerPathBase}/reexecution/set-not-found?renderMode={renderMode}");
-        await Task.Delay(5000);
         AssertNotFoundFragmentRendered();
     }
 

From b166369da335f833f83be933e91cbab86232acd4 Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Tue, 10 Jun 2025 12:40:18 +0200
Subject: [PATCH 06/28] Fix tests added with this PR.

---
 .../ServerRenderingTests/InteractivityTest.cs | 57 -----------------
 .../NoInteractivityTest.cs                    |  2 +-
 .../E2ETest/Tests/GlobalInteractivityTest.cs  | 63 +++++++++++++++++++
 .../RazorComponents/App.razor                 |  2 +-
 .../Components.WasmMinimal/Routes.razor       |  2 +-
 5 files changed, 66 insertions(+), 60 deletions(-)

diff --git a/src/Components/test/E2ETest/ServerRenderingTests/InteractivityTest.cs b/src/Components/test/E2ETest/ServerRenderingTests/InteractivityTest.cs
index 9536cbbbcee9..dd9f2a3a47e8 100644
--- a/src/Components/test/E2ETest/ServerRenderingTests/InteractivityTest.cs
+++ b/src/Components/test/E2ETest/ServerRenderingTests/InteractivityTest.cs
@@ -1448,61 +1448,4 @@ public void BrowserNavigationToNotExistingPathReExecutesTo404(string renderMode)
 
     private void Assert404ReExecuted() =>
         Browser.Equal("Welcome On Page Re-executed After Not Found Event", () => Browser.Exists(By.Id("test-info")).Text);
-
-    [Theory]
-    [InlineData(true)]
-    [InlineData(false)]
-    public void CanRenderNotFoundPage_SSR(bool streamingStarted)
-    {
-        string streamingPath = streamingStarted ? "-streaming" : "";
-        Navigate($"{ServerPathBase}/set-not-found-ssr{streamingPath}?useCustomNotFoundPage=true");
-        AssertCustomNotFoundPageRendered();
-    }
-
-    [Theory]
-    [InlineData("ServerNonPrerendered")]
-    [InlineData("WebAssemblyNonPrerendered")]
-    public void CanRenderNotFoundPage_Interactive(string renderMode)
-    {
-        Navigate($"{ServerPathBase}/set-not-found?useCustomNotFoundPage=true&renderMode={renderMode}");
-        AssertCustomNotFoundPageRendered();
-    }
-
-    private void AssertCustomNotFoundPageRendered()
-    {
-        var infoText = Browser.FindElement(By.Id("test-info")).Text;
-        Assert.Contains("Welcome On Custom Not Found Page", infoText);
-        // custom page should have a custom layout
-        var aboutLink = Browser.FindElement(By.Id("about-link")).Text;
-        Assert.Contains("About", aboutLink);
-    }
-
-    [Theory]
-    [InlineData(false)]
-    [InlineData(true)]
-    public void DoesNotReExecuteIf404WasHandled_SSR(bool streamingStarted)
-    {
-        string streamingPath = streamingStarted ? "-streaming" : "";
-        Navigate($"{ServerPathBase}/reexecution/set-not-found-ssr{streamingPath}");
-        AssertNotFoundFragmentRendered();
-    }
-
-    [Theory]
-    [InlineData("ServerNonPrerendered")]
-    [InlineData("WebAssemblyNonPrerendered")]
-    public void DoesNotReExecuteIf404WasHandled_Interactive(string renderMode)
-    {
-        Navigate($"{ServerPathBase}/reexecution/set-not-found?renderMode={renderMode}");
-        AssertNotFoundFragmentRendered();
-    }
-
-    private void AssertNotFoundFragmentRendered() =>
-        Browser.Equal("There's nothing here", () => Browser.FindElement(By.CssSelector("body > p")).Text);
-
-    [Fact]
-    public void StatusCodePagesWithReExecution()
-    {
-        Navigate($"{ServerPathBase}/reexecution/trigger-404");
-        Assert404ReExecuted();
-    }
 }
diff --git a/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs b/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
index 26399b8f556f..536eab21885a 100644
--- a/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
+++ b/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
@@ -143,7 +143,7 @@ public void DoesNotReExecuteIf404WasHandled(bool streamingStarted)
     }
 
     private void AssertNotFoundFragmentRendered() =>
-        Browser.Equal("There's nothing here", () => Browser.FindElement(By.CssSelector("body > p")).Text);
+        Browser.Equal("There's nothing here", () => Browser.FindElement(By.Id("not-found-fragment")).Text);
 
     [Fact]
     public void StatusCodePagesWithReExecution()
diff --git a/src/Components/test/E2ETest/Tests/GlobalInteractivityTest.cs b/src/Components/test/E2ETest/Tests/GlobalInteractivityTest.cs
index 0595baa95bc0..2bc20d31dee6 100644
--- a/src/Components/test/E2ETest/Tests/GlobalInteractivityTest.cs
+++ b/src/Components/test/E2ETest/Tests/GlobalInteractivityTest.cs
@@ -140,4 +140,67 @@ public void CanNavigateBetweenStaticPagesViaEnhancedNav()
         Browser.Equal("Global interactivity page: Static via attribute", () => h1.Text);
         Browser.Equal("static", () => Browser.Exists(By.Id("execution-mode")).Text);
     }
+
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public void CanRenderNotFoundPage_SSR(bool streamingStarted)
+    {
+        string streamingPath = streamingStarted ? "-streaming" : "";
+        Navigate($"{ServerPathBase}/set-not-found-ssr{streamingPath}?useCustomNotFoundPage=true");
+        AssertCustomNotFoundPageRendered();
+    }
+
+    [Theory]
+    [InlineData("ServerNonPrerendered")]
+    [InlineData("WebAssemblyNonPrerendered")]
+    public void CanRenderNotFoundPage_Interactive(string renderMode)
+    {
+        Navigate($"{ServerPathBase}/set-not-found?useCustomNotFoundPage=true&renderMode={renderMode}");
+        AssertCustomNotFoundPageRendered();
+    }
+
+    private void AssertCustomNotFoundPageRendered()
+    {
+        var infoText = Browser.FindElement(By.Id("test-info")).Text;
+        Assert.Contains("Welcome On Custom Not Found Page", infoText);
+        // custom page should have a custom layout
+        var aboutLink = Browser.FindElement(By.Id("about-link")).Text;
+        Assert.Contains("About", aboutLink);
+    }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void DoesNotReExecuteIf404WasHandled_SSR(bool streamingStarted)
+    {
+        string streamingPath = streamingStarted ? "-streaming" : "";
+        Navigate($"{ServerPathBase}/reexecution/set-not-found-ssr{streamingPath}");
+        AssertNotFoundFragmentRendered();
+    }
+
+    [Theory]
+    [InlineData("ServerNonPrerendered")]
+    [InlineData("WebAssemblyNonPrerendered")]
+    public void DoesNotReExecuteIf404WasHandled_Interactive(string renderMode)
+    {
+        Navigate($"{ServerPathBase}/reexecution/set-not-found?renderMode={renderMode}");
+        AssertNotFoundFragmentRendered();
+    }
+
+    private void AssertNotFoundFragmentRendered()
+    {
+        var body = Browser.FindElement(By.TagName("body"));
+        var notFound = Browser.FindElement(By.Id("not-found-fragment")).Text;
+        Browser.Equal("There's nothing here", () => Browser.FindElement(By.Id("not-found-fragment")).Text);
+    }
+
+    [Fact]
+    public void StatusCodePagesWithReExecution()
+    {
+        Navigate($"{ServerPathBase}/reexecution/trigger-404");
+        Assert404ReExecuted();
+    }
+    private void Assert404ReExecuted() =>
+        Browser.Equal("Welcome On Page Re-executed After Not Found Event", () => Browser.Exists(By.Id("test-info")).Text);
 }
diff --git a/src/Components/test/testassets/Components.TestServer/RazorComponents/App.razor b/src/Components/test/testassets/Components.TestServer/RazorComponents/App.razor
index a4e8ff2dff98..9912b1055e79 100644
--- a/src/Components/test/testassets/Components.TestServer/RazorComponents/App.razor
+++ b/src/Components/test/testassets/Components.TestServer/RazorComponents/App.razor
@@ -34,7 +34,7 @@
             <RouteView RouteData="@routeData" />
             <FocusOnNavigate RouteData="@routeData" Selector="[data-focus-on-navigate]" />
         </Found>
-        <NotFound>There's nothing here</NotFound>
+        <NotFound><p id="not-found-fragment">There's nothing here</p></NotFound>
     </Router>
     <script>
         // This script must come before blazor.web.js to test that
diff --git a/src/Components/test/testassets/Components.WasmMinimal/Routes.razor b/src/Components/test/testassets/Components.WasmMinimal/Routes.razor
index ca687782bc22..18226373e31c 100644
--- a/src/Components/test/testassets/Components.WasmMinimal/Routes.razor
+++ b/src/Components/test/testassets/Components.WasmMinimal/Routes.razor
@@ -27,5 +27,5 @@
         <RouteView RouteData="@routeData" />
         <FocusOnNavigate RouteData="@routeData" Selector="h1" />
     </Found>
-    <NotFound>There's nothing here</NotFound>
+    <NotFound><p id="not-found-fragment">There's nothing here</p></NotFound>
 </Router>

From 17048110d638f05c9804c9b4f8641ed5e8846032 Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Wed, 11 Jun 2025 08:20:57 +0200
Subject: [PATCH 07/28] Refactor test.

---
 .../test/E2ETest/Tests/GlobalInteractivityTest.cs           | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/src/Components/test/E2ETest/Tests/GlobalInteractivityTest.cs b/src/Components/test/E2ETest/Tests/GlobalInteractivityTest.cs
index 2bc20d31dee6..368cfab8251c 100644
--- a/src/Components/test/E2ETest/Tests/GlobalInteractivityTest.cs
+++ b/src/Components/test/E2ETest/Tests/GlobalInteractivityTest.cs
@@ -37,11 +37,7 @@ public void CanRenderNotFoundInteractive(string renderingMode, bool useCustomNot
 
         if (useCustomNotFoundPage)
         {
-            var infoText = Browser.FindElement(By.Id("test-info")).Text;
-            Assert.Contains("Welcome On Custom Not Found Page", infoText);
-            // custom page should have a custom layout
-            var aboutLink = Browser.FindElement(By.Id("about-link")).Text;
-            Assert.Contains("About", aboutLink);
+            AssertCustomNotFoundPageRendered();
         }
         else
         {

From 8184dd088e7ae3611027b05a6d92233d2937fcf1 Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Wed, 11 Jun 2025 08:40:47 +0200
Subject: [PATCH 08/28] Fix enhanced navigation tests.

---
 .../Pages/StreamingRendering/LargeStreamRendering.razor         | 2 ++
 .../Pages/StreamingRendering/StreamingRendering.razor           | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/src/Components/test/testassets/Components.TestServer/RazorComponents/Pages/StreamingRendering/LargeStreamRendering.razor b/src/Components/test/testassets/Components.TestServer/RazorComponents/Pages/StreamingRendering/LargeStreamRendering.razor
index 9b6d199c76ee..99ea274e45d2 100644
--- a/src/Components/test/testassets/Components.TestServer/RazorComponents/Pages/StreamingRendering/LargeStreamRendering.razor
+++ b/src/Components/test/testassets/Components.TestServer/RazorComponents/Pages/StreamingRendering/LargeStreamRendering.razor
@@ -2,6 +2,8 @@
 
 @attribute [StreamRendering]
 
+<PageTitle>Streaming</PageTitle>
+
 <h1>Streaming Rendering</h1>
 
 <NotEnabledStreamingRenderingComponent UseLargeItems="true"/>
diff --git a/src/Components/test/testassets/Components.TestServer/RazorComponents/Pages/StreamingRendering/StreamingRendering.razor b/src/Components/test/testassets/Components.TestServer/RazorComponents/Pages/StreamingRendering/StreamingRendering.razor
index 0bd36fd4817c..64f85052ae2d 100644
--- a/src/Components/test/testassets/Components.TestServer/RazorComponents/Pages/StreamingRendering/StreamingRendering.razor
+++ b/src/Components/test/testassets/Components.TestServer/RazorComponents/Pages/StreamingRendering/StreamingRendering.razor
@@ -2,6 +2,8 @@
 
 @attribute [StreamRendering]
 
+<PageTitle>Streaming</PageTitle>
+
 <h1>Streaming Rendering</h1>
 
 <NotEnabledStreamingRenderingComponent />

From 707683f0489016fafb353416cc6cceb2cc4f4df0 Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Wed, 11 Jun 2025 09:07:35 +0200
Subject: [PATCH 09/28] Markers for: `Router`, `Found`, `RouteView`,
 `RoutedPage` are now present in the response.

---
 .../Mvc.FunctionalTests/ComponentRenderingFunctionalTests.cs    | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/Mvc/test/Mvc.FunctionalTests/ComponentRenderingFunctionalTests.cs b/src/Mvc/test/Mvc.FunctionalTests/ComponentRenderingFunctionalTests.cs
index d319827f9a33..11ed2cd95595 100644
--- a/src/Mvc/test/Mvc.FunctionalTests/ComponentRenderingFunctionalTests.cs
+++ b/src/Mvc/test/Mvc.FunctionalTests/ComponentRenderingFunctionalTests.cs
@@ -89,7 +89,7 @@ public async Task Renders_RoutingComponent_UsingRazorComponents_Prerenderer()
         await response.AssertStatusCodeAsync(HttpStatusCode.OK);
         var content = await response.Content.ReadAsStringAsync();
 
-        AssertComponent("Router component\n<p>Routed successfully</p>", "Routing", content);
+        AssertComponent("Router component\n<!--bl:3--><!--bl:4--><!--bl:5--><!--bl:6--><p>Routed successfully</p><!--/bl:6--><!--/bl:5--><!--/bl:4--><!--/bl:3-->", "Routing", content);
     }
 
     [Fact]

From 79c95e9d2f0a164ca255f162bdf4dbe789a4ebf5 Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Thu, 12 Jun 2025 13:27:27 +0200
Subject: [PATCH 10/28] Client streams-in the NotFoundPage if it's provided to
 the Router.

---
 .../Components/src/NavigationManager.cs       |  6 +--
 .../Components/src/PublicAPI.Unshipped.txt    |  4 +-
 .../src/Routing/NotFoundEventArgs.cs          | 18 +++++++-
 .../EndpointHtmlRenderer.EventDispatch.cs     | 36 ++++++++++++---
 .../EndpointHtmlRenderer.Streaming.cs         |  9 ++++
 .../src/Rendering/EndpointHtmlRenderer.cs     |  5 +-
 .../src/Rendering/StreamingRendering.ts       | 46 +++++++++++++++++++
 7 files changed, 110 insertions(+), 14 deletions(-)

diff --git a/src/Components/Components/src/NavigationManager.cs b/src/Components/Components/src/NavigationManager.cs
index 25d6da65b94e..987482ece5a7 100644
--- a/src/Components/Components/src/NavigationManager.cs
+++ b/src/Components/Components/src/NavigationManager.cs
@@ -54,8 +54,6 @@ public event EventHandler<NotFoundEventArgs> OnNotFound
 
     private EventHandler<NotFoundEventArgs>? _notFound;
 
-    private static readonly NotFoundEventArgs _notFoundEventArgs = new NotFoundEventArgs();
-
     // For the baseUri it's worth storing as a System.Uri so we can do operations
     // on that type. System.Uri gives us access to the original string anyway.
     private Uri? _baseUri;
@@ -63,6 +61,8 @@ public event EventHandler<NotFoundEventArgs> OnNotFound
     // The URI. Always represented an absolute URI.
     private string? _uri;
     private bool _isInitialized;
+    internal string NotFoundPageRoute { get; set; } = string.Empty;
+    internal Type NotFoundPageType { get; set; } = default!;
 
     /// <summary>
     /// Gets or sets the current base URI. The <see cref="BaseUri" /> is always represented as an absolute URI in string form with trailing slash.
@@ -212,7 +212,7 @@ private void NotFoundCore()
         }
         else
         {
-            _notFound.Invoke(this, _notFoundEventArgs);
+            _notFound.Invoke(this, new NotFoundEventArgs(NotFoundPageRoute, NotFoundPageType)); // ToDo: sending only the type would be enough and then we would get the route later
         }
     }
 
diff --git a/src/Components/Components/src/PublicAPI.Unshipped.txt b/src/Components/Components/src/PublicAPI.Unshipped.txt
index 5eb52d2c330e..bb9da6e52ead 100644
--- a/src/Components/Components/src/PublicAPI.Unshipped.txt
+++ b/src/Components/Components/src/PublicAPI.Unshipped.txt
@@ -6,7 +6,9 @@ Microsoft.AspNetCore.Components.NavigationManager.OnNotFound -> System.EventHand
 Microsoft.AspNetCore.Components.NavigationManager.NotFound() -> void
 Microsoft.AspNetCore.Components.Routing.IHostEnvironmentNavigationManager.Initialize(string! baseUri, string! uri, System.Func<string!, System.Threading.Tasks.Task!>! onNavigateTo) -> void
 Microsoft.AspNetCore.Components.Routing.NotFoundEventArgs
-Microsoft.AspNetCore.Components.Routing.NotFoundEventArgs.NotFoundEventArgs() -> void
+Microsoft.AspNetCore.Components.Routing.NotFoundEventArgs.NotFoundEventArgs(string! url, System.Type! notFoundPageType) -> void
+Microsoft.AspNetCore.Components.Routing.NotFoundEventArgs.NotFoundPageType.get -> System.Type!
+Microsoft.AspNetCore.Components.Routing.NotFoundEventArgs.Path.get -> string!
 Microsoft.AspNetCore.Components.Infrastructure.ComponentStatePersistenceManager.ComponentStatePersistenceManager(Microsoft.Extensions.Logging.ILogger<Microsoft.AspNetCore.Components.Infrastructure.ComponentStatePersistenceManager!>! logger, System.IServiceProvider! serviceProvider) -> void
 Microsoft.AspNetCore.Components.Infrastructure.ComponentStatePersistenceManager.SetPlatformRenderMode(Microsoft.AspNetCore.Components.IComponentRenderMode! renderMode) -> void
 Microsoft.AspNetCore.Components.Infrastructure.RegisterPersistentComponentStateServiceCollectionExtensions
diff --git a/src/Components/Components/src/Routing/NotFoundEventArgs.cs b/src/Components/Components/src/Routing/NotFoundEventArgs.cs
index 637bfd442b70..07eca1a9c906 100644
--- a/src/Components/Components/src/Routing/NotFoundEventArgs.cs
+++ b/src/Components/Components/src/Routing/NotFoundEventArgs.cs
@@ -8,9 +8,23 @@ namespace Microsoft.AspNetCore.Components.Routing;
 /// </summary>
 public sealed class NotFoundEventArgs : EventArgs
 {
+    /// <summary>
+    /// Gets the path of NotFoundPage.
+    /// </summary>
+    public string Path { get; }
+
+    /// <summary>
+    /// Gets the type of NotFoundPage component.
+    /// </summary>
+    public Type NotFoundPageType { get; }
+
     /// <summary>
     /// Initializes a new instance of <see cref="NotFoundEventArgs" />.
     /// </summary>
-    public NotFoundEventArgs()
-    { }
+    public NotFoundEventArgs(string url, Type notFoundPageType)
+    {
+        Path = url;
+        NotFoundPageType = notFoundPageType;
+    }
+
 }
diff --git a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
index 4f308c7984f7..8c60a17f00cf 100644
--- a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
+++ b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
@@ -4,6 +4,7 @@
 using Microsoft.AspNetCore.Components.Endpoints.Rendering;
 using Microsoft.AspNetCore.Components.Rendering;
 using Microsoft.AspNetCore.Components.RenderTree;
+using Microsoft.AspNetCore.Components.Routing;
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.WebUtilities;
 using Microsoft.Extensions.DependencyInjection;
@@ -77,21 +78,42 @@ private Task ReturnErrorResponse(string detailedMessage)
             : Task.CompletedTask;
     }
 
-    private void SetNotFoundResponse(object? sender, EventArgs args)
+    private async Task SetNotFoundResponseAsync(string baseUri, NotFoundEventArgs args)
     {
         if (_httpContext.Response.HasStarted)
         {
-            // We're expecting the Router to continue streaming the NotFound contents
+            if (args.NotFoundPageType == null || string.IsNullOrEmpty(args.Path))
+            {
+                throw new InvalidOperationException("The NotFoundPageType and Path must be specified in the NotFoundEventArgs to render NotFoundPage when the response has started.");
+            }
+            var instance = Activator.CreateInstance(args.NotFoundPageType) as IComponent;
+            if (instance == null)
+            {
+                throw new InvalidOperationException($"The type {args.NotFoundPageType.FullName} does not implement IComponent.");
+            }
+            if (_notFoundComponentId == -1)
+            {
+                _notFoundComponentId = AssignRootComponentId(instance);
+            }
+            if (string.IsNullOrEmpty(_notFoundUrl))
+            {
+                _notFoundUrl = $"{baseUri}{args.Path.TrimStart('/')}";
+            }
+            var defaultBufferSize = 16 * 1024;
+            await using var writer = new HttpResponseStreamWriter(_httpContext.Response.Body, Encoding.UTF8, defaultBufferSize, ArrayPool<byte>.Shared, ArrayPool<char>.Shared);
+            using var bufferWriter = new BufferedTextWriter(writer);
+            HandleNotFoundAfterResponseStarted(bufferWriter, _httpContext, _notFoundUrl, _notFoundComponentId);
+            await bufferWriter.FlushAsync();
         }
         else
         {
             _httpContext.Response.StatusCode = StatusCodes.Status404NotFound;
-
-            // When the application triggers a NotFound event, we continue rendering the current batch.
-            // However, after completing this batch, we do not want to process any further UI updates,
-            // as we are going to return a 404 status and discard the UI updates generated so far.
-            SignalRendererToFinishRendering();
         }
+
+        // When the application triggers a NotFound event, we continue rendering the current batch.
+        // However, after completing this batch, we do not want to process any further UI updates,
+        // as we are going to return a 404 status and discard the UI updates generated so far.
+        SignalRendererToFinishRendering();
     }
 
     private async Task OnNavigateTo(string uri)
diff --git a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.Streaming.cs b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.Streaming.cs
index c17f7cd53555..f76b8c873f55 100644
--- a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.Streaming.cs
+++ b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.Streaming.cs
@@ -226,6 +226,15 @@ private static void HandleExceptionAfterResponseStarted(HttpContext httpContext,
         writer.Write("</template><blazor-ssr-end></blazor-ssr-end></blazor-ssr>");
     }
 
+    private static void HandleNotFoundAfterResponseStarted(TextWriter writer, HttpContext httpContext, string notFoundUrl, int componentId)
+    {
+        writer.Write("<blazor-ssr><template type=\"not-found\"");
+        writer.Write($" componentId=\"{componentId}\"");
+        writer.Write(">");
+        writer.Write(HtmlEncoder.Default.Encode(OpaqueRedirection.CreateProtectedRedirectionUrl(httpContext, notFoundUrl)));
+        writer.Write("</template><blazor-ssr-end></blazor-ssr-end></blazor-ssr>");
+    }
+
     private static void HandleNavigationAfterResponseStarted(TextWriter writer, HttpContext httpContext, string destinationUrl)
     {
         writer.Write("<blazor-ssr><template type=\"redirection\"");
diff --git a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs
index cc491759cb20..e96b88971ad5 100644
--- a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs
+++ b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs
@@ -52,6 +52,9 @@ internal partial class EndpointHtmlRenderer : StaticHtmlRenderer, IComponentPrer
     // wait for the non-streaming tasks (these ones), then start streaming until full quiescence.
     private readonly List<Task> _nonStreamingPendingTasks = new();
 
+    private int _notFoundComponentId = -1;
+    private string _notFoundUrl = string.Empty;
+
     public EndpointHtmlRenderer(IServiceProvider serviceProvider, ILoggerFactory loggerFactory)
         : base(serviceProvider, loggerFactory)
     {
@@ -85,7 +88,7 @@ internal async Task InitializeStandardComponentServicesAsync(
 
         if (navigationManager != null)
         {
-            navigationManager.OnNotFound += SetNotFoundResponse;
+            navigationManager.OnNotFound += async (sender, args) => await SetNotFoundResponseAsync(navigationManager.BaseUri, args);
         }
 
         var authenticationStateProvider = httpContext.RequestServices.GetService<AuthenticationStateProvider>();
diff --git a/src/Components/Web.JS/src/Rendering/StreamingRendering.ts b/src/Components/Web.JS/src/Rendering/StreamingRendering.ts
index 06c650c2f99c..6e2b65373a54 100644
--- a/src/Components/Web.JS/src/Rendering/StreamingRendering.ts
+++ b/src/Components/Web.JS/src/Rendering/StreamingRendering.ts
@@ -8,6 +8,8 @@ import { synchronizeDomContent } from './DomMerging/DomSync';
 
 let enableDomPreservation = true;
 let navigationEnhancementCallbacks: NavigationEnhancementCallbacks;
+let currentNotFoundRenderAbortController: AbortController | null;
+const acceptHeader = 'text/html; blazor-enhanced-nav=on';
 
 export function attachStreamingRenderingListener(options: SsrStartOptions | undefined, callbacks: NavigationEnhancementCallbacks) {
   navigationEnhancementCallbacks = callbacks;
@@ -75,6 +77,34 @@ class BlazorStreamingUpdate extends HTMLElement {
                 }
               }
               break;
+            case 'not-found':
+              const componentId = node.getAttribute('componentId');
+              if (!componentId) {
+                console.error('Streaming content for not-found response does not have a componentId attribute.');
+                break;
+              }
+              wrapDocumentInStreamingMarkers(componentId);
+              const notFoundUrl = toAbsoluteUri(node.content.textContent!);
+
+              if (componentId) {
+                currentNotFoundRenderAbortController?.abort();
+                currentNotFoundRenderAbortController = new AbortController();
+                const abortSignal = currentNotFoundRenderAbortController.signal;
+                fetch(notFoundUrl, Object.assign(<RequestInit>{
+                  signal: abortSignal,
+                  mode: 'no-cors',
+                  headers: {
+                    'accept': acceptHeader,
+                  },
+                }, undefined))
+                  .then(response => response.text())
+                  .then(html => {
+                    const docFrag = document.createRange().createContextualFragment(html);
+                    insertStreamingContentIntoDocument(componentId, docFrag);
+                  })
+                  .catch(error => console.error('Failed to fetch not-found content:', error));
+              }
+              break;
             case 'error':
               // This is kind of brutal but matches what happens without progressive enhancement
               replaceDocumentWithPlainText(node.content.textContent || 'Error');
@@ -86,6 +116,22 @@ class BlazorStreamingUpdate extends HTMLElement {
   }
 }
 
+function wrapDocumentInStreamingMarkers(componentIdAsString: string): void {
+  // Add a comment before the <head>
+  const markerStart = document.createComment(`bl:${componentIdAsString}`);
+  const head = document.querySelector('head');
+  document.documentElement.insertBefore(markerStart, head);
+
+  // Add a comment after the <body> or after the <head> if <body> is null
+  const markerEnd = document.createComment(`/bl:${componentIdAsString}`);
+  const body = document.querySelector('body');
+  if (body && body.parentNode) {
+    body.parentNode.insertBefore(markerEnd, body.nextSibling);
+  } else if (head && head.parentNode) {
+    head.parentNode.insertBefore(markerEnd, head.nextSibling);
+  }
+}
+
 function insertStreamingContentIntoDocument(componentIdAsString: string, docFrag: DocumentFragment): void {
   const markers = findStreamingMarkers(componentIdAsString);
   if (markers) {

From 82db52fd974e730d8c92b87f7556d1bc6251e1c1 Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Thu, 12 Jun 2025 16:48:13 +0200
Subject: [PATCH 11/28] Update: client rendering the NotFoundPage if it's
 provided to the Router or when re-execution middleware is set.

---
 .../Components/src/NavigationManager.cs       |  3 +-
 .../Components/src/PublicAPI.Unshipped.txt    |  3 +-
 .../src/Routing/NotFoundEventArgs.cs          |  8 +-
 .../Components/src/Routing/Router.cs          |  6 ++
 .../EndpointHtmlRenderer.EventDispatch.cs     | 34 ++++---
 .../EndpointHtmlRenderer.Streaming.cs         | 11 ++-
 .../src/Rendering/EndpointHtmlRenderer.cs     |  1 -
 .../src/Rendering/StreamingRendering.ts       | 97 ++++++-------------
 .../src/Services/NavigationEnhancement.ts     | 12 ++-
 .../StatusCodePagesExtensions.cs              |  9 +-
 .../StatusCodePagesMiddleware.cs              |  6 ++
 .../StatusCodePage/StatusCodePagesOptions.cs  |  2 +
 12 files changed, 84 insertions(+), 108 deletions(-)

diff --git a/src/Components/Components/src/NavigationManager.cs b/src/Components/Components/src/NavigationManager.cs
index 987482ece5a7..1900b91c629b 100644
--- a/src/Components/Components/src/NavigationManager.cs
+++ b/src/Components/Components/src/NavigationManager.cs
@@ -62,7 +62,6 @@ public event EventHandler<NotFoundEventArgs> OnNotFound
     private string? _uri;
     private bool _isInitialized;
     internal string NotFoundPageRoute { get; set; } = string.Empty;
-    internal Type NotFoundPageType { get; set; } = default!;
 
     /// <summary>
     /// Gets or sets the current base URI. The <see cref="BaseUri" /> is always represented as an absolute URI in string form with trailing slash.
@@ -212,7 +211,7 @@ private void NotFoundCore()
         }
         else
         {
-            _notFound.Invoke(this, new NotFoundEventArgs(NotFoundPageRoute, NotFoundPageType)); // ToDo: sending only the type would be enough and then we would get the route later
+            _notFound.Invoke(this, new NotFoundEventArgs(NotFoundPageRoute));
         }
     }
 
diff --git a/src/Components/Components/src/PublicAPI.Unshipped.txt b/src/Components/Components/src/PublicAPI.Unshipped.txt
index bb9da6e52ead..07e51aca6bd3 100644
--- a/src/Components/Components/src/PublicAPI.Unshipped.txt
+++ b/src/Components/Components/src/PublicAPI.Unshipped.txt
@@ -6,8 +6,7 @@ Microsoft.AspNetCore.Components.NavigationManager.OnNotFound -> System.EventHand
 Microsoft.AspNetCore.Components.NavigationManager.NotFound() -> void
 Microsoft.AspNetCore.Components.Routing.IHostEnvironmentNavigationManager.Initialize(string! baseUri, string! uri, System.Func<string!, System.Threading.Tasks.Task!>! onNavigateTo) -> void
 Microsoft.AspNetCore.Components.Routing.NotFoundEventArgs
-Microsoft.AspNetCore.Components.Routing.NotFoundEventArgs.NotFoundEventArgs(string! url, System.Type! notFoundPageType) -> void
-Microsoft.AspNetCore.Components.Routing.NotFoundEventArgs.NotFoundPageType.get -> System.Type!
+Microsoft.AspNetCore.Components.Routing.NotFoundEventArgs.NotFoundEventArgs(string! url) -> void
 Microsoft.AspNetCore.Components.Routing.NotFoundEventArgs.Path.get -> string!
 Microsoft.AspNetCore.Components.Infrastructure.ComponentStatePersistenceManager.ComponentStatePersistenceManager(Microsoft.Extensions.Logging.ILogger<Microsoft.AspNetCore.Components.Infrastructure.ComponentStatePersistenceManager!>! logger, System.IServiceProvider! serviceProvider) -> void
 Microsoft.AspNetCore.Components.Infrastructure.ComponentStatePersistenceManager.SetPlatformRenderMode(Microsoft.AspNetCore.Components.IComponentRenderMode! renderMode) -> void
diff --git a/src/Components/Components/src/Routing/NotFoundEventArgs.cs b/src/Components/Components/src/Routing/NotFoundEventArgs.cs
index 07eca1a9c906..e1e81e5cfc82 100644
--- a/src/Components/Components/src/Routing/NotFoundEventArgs.cs
+++ b/src/Components/Components/src/Routing/NotFoundEventArgs.cs
@@ -13,18 +13,12 @@ public sealed class NotFoundEventArgs : EventArgs
     /// </summary>
     public string Path { get; }
 
-    /// <summary>
-    /// Gets the type of NotFoundPage component.
-    /// </summary>
-    public Type NotFoundPageType { get; }
-
     /// <summary>
     /// Initializes a new instance of <see cref="NotFoundEventArgs" />.
     /// </summary>
-    public NotFoundEventArgs(string url, Type notFoundPageType)
+    public NotFoundEventArgs(string url)
     {
         Path = url;
-        NotFoundPageType = notFoundPageType;
     }
 
 }
diff --git a/src/Components/Components/src/Routing/Router.cs b/src/Components/Components/src/Routing/Router.cs
index 0f815973a561..dbf8477a0a7f 100644
--- a/src/Components/Components/src/Routing/Router.cs
+++ b/src/Components/Components/src/Routing/Router.cs
@@ -156,6 +156,12 @@ public async Task SetParametersAsync(ParameterView parameters)
                 throw new InvalidOperationException($"The type {NotFoundPage.FullName} " +
                     $"does not have a {typeof(RouteAttribute).FullName} applied to it.");
             }
+
+            var routeAttribute = (RouteAttribute)routeAttributes[0];
+            if (routeAttribute.Template != null)
+            {
+                NavigationManager.NotFoundPageRoute = routeAttribute.Template;
+            }
         }
 
         if (!_onNavigateCalled)
diff --git a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
index 8c60a17f00cf..75c933521dbd 100644
--- a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
+++ b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Components.Endpoints.Rendering;
 using Microsoft.AspNetCore.Components.Rendering;
 using Microsoft.AspNetCore.Components.RenderTree;
@@ -82,27 +83,14 @@ private async Task SetNotFoundResponseAsync(string baseUri, NotFoundEventArgs ar
     {
         if (_httpContext.Response.HasStarted)
         {
-            if (args.NotFoundPageType == null || string.IsNullOrEmpty(args.Path))
-            {
-                throw new InvalidOperationException("The NotFoundPageType and Path must be specified in the NotFoundEventArgs to render NotFoundPage when the response has started.");
-            }
-            var instance = Activator.CreateInstance(args.NotFoundPageType) as IComponent;
-            if (instance == null)
-            {
-                throw new InvalidOperationException($"The type {args.NotFoundPageType.FullName} does not implement IComponent.");
-            }
-            if (_notFoundComponentId == -1)
-            {
-                _notFoundComponentId = AssignRootComponentId(instance);
-            }
             if (string.IsNullOrEmpty(_notFoundUrl))
             {
-                _notFoundUrl = $"{baseUri}{args.Path.TrimStart('/')}";
+                _notFoundUrl = GetNotFoundUrl(baseUri, args);
             }
             var defaultBufferSize = 16 * 1024;
             await using var writer = new HttpResponseStreamWriter(_httpContext.Response.Body, Encoding.UTF8, defaultBufferSize, ArrayPool<byte>.Shared, ArrayPool<char>.Shared);
             using var bufferWriter = new BufferedTextWriter(writer);
-            HandleNotFoundAfterResponseStarted(bufferWriter, _httpContext, _notFoundUrl, _notFoundComponentId);
+            HandleNotFoundAfterResponseStarted(bufferWriter, _httpContext, _notFoundUrl);
             await bufferWriter.FlushAsync();
         }
         else
@@ -116,6 +104,22 @@ private async Task SetNotFoundResponseAsync(string baseUri, NotFoundEventArgs ar
         SignalRendererToFinishRendering();
     }
 
+    private string GetNotFoundUrl(string baseUri, NotFoundEventArgs args)
+    {
+        string path = args.Path;
+        if (string.IsNullOrEmpty(path))
+        {
+            var pathFormat = _httpContext.Items[nameof(StatusCodePagesOptions)] as string;
+            if (string.IsNullOrEmpty(pathFormat))
+            {
+                throw new InvalidOperationException("The NotFoundPage route must be specified or re-execution middleware has to be set to render NotFoundPage when the response has started.");
+            }
+
+            path = pathFormat;
+        }
+        return $"{baseUri}{path.TrimStart('/')}";
+    }
+
     private async Task OnNavigateTo(string uri)
     {
         if (_httpContext.Response.HasStarted)
diff --git a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.Streaming.cs b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.Streaming.cs
index f76b8c873f55..e78a938fc9d6 100644
--- a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.Streaming.cs
+++ b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.Streaming.cs
@@ -226,19 +226,20 @@ private static void HandleExceptionAfterResponseStarted(HttpContext httpContext,
         writer.Write("</template><blazor-ssr-end></blazor-ssr-end></blazor-ssr>");
     }
 
-    private static void HandleNotFoundAfterResponseStarted(TextWriter writer, HttpContext httpContext, string notFoundUrl, int componentId)
+    private static void HandleNotFoundAfterResponseStarted(TextWriter writer, HttpContext httpContext, string notFoundUrl)
     {
         writer.Write("<blazor-ssr><template type=\"not-found\"");
-        writer.Write($" componentId=\"{componentId}\"");
-        writer.Write(">");
-        writer.Write(HtmlEncoder.Default.Encode(OpaqueRedirection.CreateProtectedRedirectionUrl(httpContext, notFoundUrl)));
-        writer.Write("</template><blazor-ssr-end></blazor-ssr-end></blazor-ssr>");
+        WriteResponseTemplate(writer, httpContext, notFoundUrl);
     }
 
     private static void HandleNavigationAfterResponseStarted(TextWriter writer, HttpContext httpContext, string destinationUrl)
     {
         writer.Write("<blazor-ssr><template type=\"redirection\"");
+        WriteResponseTemplate(writer, httpContext, destinationUrl);
+    }
 
+    private static void WriteResponseTemplate(TextWriter writer, HttpContext httpContext, string destinationUrl)
+    {
         if (string.Equals(httpContext.Request.Method, "POST", StringComparison.OrdinalIgnoreCase))
         {
             writer.Write(" from=\"form-post\"");
diff --git a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs
index e96b88971ad5..a306680452a1 100644
--- a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs
+++ b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs
@@ -52,7 +52,6 @@ internal partial class EndpointHtmlRenderer : StaticHtmlRenderer, IComponentPrer
     // wait for the non-streaming tasks (these ones), then start streaming until full quiescence.
     private readonly List<Task> _nonStreamingPendingTasks = new();
 
-    private int _notFoundComponentId = -1;
     private string _notFoundUrl = string.Empty;
 
     public EndpointHtmlRenderer(IServiceProvider serviceProvider, ILoggerFactory loggerFactory)
diff --git a/src/Components/Web.JS/src/Rendering/StreamingRendering.ts b/src/Components/Web.JS/src/Rendering/StreamingRendering.ts
index 6e2b65373a54..cefc8829f0c8 100644
--- a/src/Components/Web.JS/src/Rendering/StreamingRendering.ts
+++ b/src/Components/Web.JS/src/Rendering/StreamingRendering.ts
@@ -8,8 +8,6 @@ import { synchronizeDomContent } from './DomMerging/DomSync';
 
 let enableDomPreservation = true;
 let navigationEnhancementCallbacks: NavigationEnhancementCallbacks;
-let currentNotFoundRenderAbortController: AbortController | null;
-const acceptHeader = 'text/html; blazor-enhanced-nav=on';
 
 export function attachStreamingRenderingListener(options: SsrStartOptions | undefined, callbacks: NavigationEnhancementCallbacks) {
   navigationEnhancementCallbacks = callbacks;
@@ -49,61 +47,10 @@ class BlazorStreamingUpdate extends HTMLElement {
         } else {
           switch (node.getAttribute('type')) {
             case 'redirection':
-              // We use 'replace' here because it's closest to the non-progressively-enhanced behavior, and will make the most sense
-              // if the async delay was very short, as the user would not perceive having been on the intermediate page.
-              const destinationUrl = toAbsoluteUri(node.content.textContent!);
-              const isFormPost = node.getAttribute('from') === 'form-post';
-              const isEnhancedNav = node.getAttribute('enhanced') === 'true';
-              if (isEnhancedNav && isWithinBaseUriSpace(destinationUrl)) {
-                // At this point the destinationUrl might be an opaque URL so we don't know whether it's internal/external or
-                // whether it's even going to the same URL we're currently on. So we don't know how to update the history.
-                // Defer that until the redirection is resolved by performEnhancedPageLoad.
-                const treatAsRedirectionFromMethod = isFormPost ? 'post' : 'get';
-                const fetchOptions = undefined;
-                performEnhancedPageLoad(destinationUrl, /* interceptedLink */ false, fetchOptions, treatAsRedirectionFromMethod);
-              } else {
-                if (isFormPost) {
-                  // The URL is not yet updated. Push a whole new entry so that 'back' goes back to the pre-redirection location.
-                  // WARNING: The following check to avoid duplicating history entries won't work if the redirection is to an opaque URL.
-                  // We could change the server-side logic to return URLs in plaintext if they match the current request URL already,
-                  // but it's arguably easier to understand that history non-duplication only works for enhanced nav, which is also the
-                  // case for non-streaming responses.
-                  if (destinationUrl !== location.href) {
-                    location.assign(destinationUrl);
-                  }
-                } else {
-                  // The URL was already updated on the original link click. Replace so that 'back' goes to the pre-redirection location.
-                  location.replace(destinationUrl);
-                }
-              }
+              redirect(node, false);
               break;
             case 'not-found':
-              const componentId = node.getAttribute('componentId');
-              if (!componentId) {
-                console.error('Streaming content for not-found response does not have a componentId attribute.');
-                break;
-              }
-              wrapDocumentInStreamingMarkers(componentId);
-              const notFoundUrl = toAbsoluteUri(node.content.textContent!);
-
-              if (componentId) {
-                currentNotFoundRenderAbortController?.abort();
-                currentNotFoundRenderAbortController = new AbortController();
-                const abortSignal = currentNotFoundRenderAbortController.signal;
-                fetch(notFoundUrl, Object.assign(<RequestInit>{
-                  signal: abortSignal,
-                  mode: 'no-cors',
-                  headers: {
-                    'accept': acceptHeader,
-                  },
-                }, undefined))
-                  .then(response => response.text())
-                  .then(html => {
-                    const docFrag = document.createRange().createContextualFragment(html);
-                    insertStreamingContentIntoDocument(componentId, docFrag);
-                  })
-                  .catch(error => console.error('Failed to fetch not-found content:', error));
-              }
+              redirect(node, true);
               break;
             case 'error':
               // This is kind of brutal but matches what happens without progressive enhancement
@@ -116,19 +63,33 @@ class BlazorStreamingUpdate extends HTMLElement {
   }
 }
 
-function wrapDocumentInStreamingMarkers(componentIdAsString: string): void {
-  // Add a comment before the <head>
-  const markerStart = document.createComment(`bl:${componentIdAsString}`);
-  const head = document.querySelector('head');
-  document.documentElement.insertBefore(markerStart, head);
-
-  // Add a comment after the <body> or after the <head> if <body> is null
-  const markerEnd = document.createComment(`/bl:${componentIdAsString}`);
-  const body = document.querySelector('body');
-  if (body && body.parentNode) {
-    body.parentNode.insertBefore(markerEnd, body.nextSibling);
-  } else if (head && head.parentNode) {
-    head.parentNode.insertBefore(markerEnd, head.nextSibling);
+function redirect(node: HTMLTemplateElement, changeUrl: boolean): void {
+  // We use 'replace' here because it's closest to the non-progressively-enhanced behavior, and will make the most sense
+ // if the async delay was very short, as the user would not perceive having been on the intermediate page.
+ const destinationUrl = toAbsoluteUri(node.content.textContent!);
+ const isFormPost = node.getAttribute('from') === 'form-post';
+ const isEnhancedNav = node.getAttribute('enhanced') === 'true';
+ if (isEnhancedNav && isWithinBaseUriSpace(destinationUrl)) {
+   // At this point the destinationUrl might be an opaque URL so we don't know whether it's internal/external or
+   // whether it's even going to the same URL we're currently on. So we don't know how to update the history.
+   // Defer that until the redirection is resolved by performEnhancedPageLoad.
+   const treatAsRedirectionFromMethod = isFormPost ? 'post' : 'get';
+   const fetchOptions = undefined;
+   performEnhancedPageLoad(destinationUrl, /* interceptedLink */ false, fetchOptions, treatAsRedirectionFromMethod, changeUrl);
+ } else {
+   if (isFormPost) {
+     // The URL is not yet updated. Push a whole new entry so that 'back' goes back to the pre-redirection location.
+     // WARNING: The following check to avoid duplicating history entries won't work if the redirection is to an opaque URL.
+     // We could change the server-side logic to return URLs in plaintext if they match the current request URL already,
+     // but it's arguably easier to understand that history non-duplication only works for enhanced nav, which is also the
+     // case for non-streaming responses.
+     if (destinationUrl !== location.href) {
+       location.assign(destinationUrl);
+     }
+   } else {
+     // The URL was already updated on the original link click. Replace so that 'back' goes to the pre-redirection location.
+     location.replace(destinationUrl);
+   }
   }
 }
 
diff --git a/src/Components/Web.JS/src/Services/NavigationEnhancement.ts b/src/Components/Web.JS/src/Services/NavigationEnhancement.ts
index 9965f2b727e2..e029c77178dd 100644
--- a/src/Components/Web.JS/src/Services/NavigationEnhancement.ts
+++ b/src/Components/Web.JS/src/Services/NavigationEnhancement.ts
@@ -192,7 +192,7 @@ function onDocumentSubmit(event: SubmitEvent) {
   }
 }
 
-export async function performEnhancedPageLoad(internalDestinationHref: string, interceptedLink: boolean, fetchOptions?: RequestInit, treatAsRedirectionFromMethod?: 'get' | 'post') {
+export async function performEnhancedPageLoad(internalDestinationHref: string, interceptedLink: boolean, fetchOptions?: RequestInit, treatAsRedirectionFromMethod?: 'get' | 'post', changeUrl: boolean = true) {
   performingEnhancedPageLoad = true;
 
   // First, stop any preceding enhanced page load
@@ -257,7 +257,7 @@ export async function performEnhancedPageLoad(internalDestinationHref: string, i
       // For 301/302/etc redirections to internal URLs, the browser will already have followed the chain of redirections
       // to the end, and given us the final content. We do still need to update the current URL to match the final location,
       // then let the rest of enhanced nav logic run to patch the new content into the DOM.
-      if (response.redirected || treatAsRedirectionFromMethod) {
+      if (changeUrl && (response.redirected || treatAsRedirectionFromMethod)) {
         const treatAsGet = treatAsRedirectionFromMethod ? (treatAsRedirectionFromMethod === 'get') : isGetRequest;
         if (treatAsGet) {
           // For gets, the intermediate (redirecting) URL is already in the address bar, so we have to use 'replace'
@@ -274,12 +274,12 @@ export async function performEnhancedPageLoad(internalDestinationHref: string, i
 
       // For enhanced nav redirecting to an external URL, we'll get a special Blazor-specific redirection command
       const externalRedirectionUrl = response.headers.get('blazor-enhanced-nav-redirect-location');
-      if (externalRedirectionUrl) {
+      if (changeUrl && externalRedirectionUrl) {
         location.replace(externalRedirectionUrl);
         return;
       }
 
-      if (!response.redirected && !isGetRequest && isSuccessResponse) {
+      if (changeUrl && !response.redirected && !isGetRequest && isSuccessResponse) {
         // If this is the result of a form post that didn't trigger a redirection.
         if (!isForSamePath(response.url, currentContentUrl)) {
           // In this case we don't want to push the currentContentUrl to the history stack because we don't know if this is a location
@@ -296,7 +296,9 @@ export async function performEnhancedPageLoad(internalDestinationHref: string, i
       }
 
       // Set the currentContentUrl to the location of the last completed navigation.
-      currentContentUrl = response.url;
+      if (changeUrl) {
+        currentContentUrl = response.url;
+      }
 
       const responseContentType = response.headers.get('content-type');
       if (responseContentType?.startsWith('text/html') && initialContent) {
diff --git a/src/Middleware/Diagnostics/src/StatusCodePage/StatusCodePagesExtensions.cs b/src/Middleware/Diagnostics/src/StatusCodePage/StatusCodePagesExtensions.cs
index 6aa6843995b8..952753c69830 100644
--- a/src/Middleware/Diagnostics/src/StatusCodePage/StatusCodePagesExtensions.cs
+++ b/src/Middleware/Diagnostics/src/StatusCodePage/StatusCodePagesExtensions.cs
@@ -187,9 +187,11 @@ public static IApplicationBuilder UseStatusCodePagesWithReExecute(
             {
                 var newNext = RerouteHelper.Reroute(app, routeBuilder, next);
                 return new StatusCodePagesMiddleware(next,
-                    Options.Create(new StatusCodePagesOptions() {
+                    Options.Create(new StatusCodePagesOptions()
+                    {
                         HandleAsync = CreateHandler(pathFormat, queryFormat, newNext),
-                        CreateScopeForErrors = createScopeForErrors
+                        CreateScopeForErrors = createScopeForErrors,
+                        PathFormat = pathFormat
                     })).Invoke;
             });
         }
@@ -197,7 +199,8 @@ public static IApplicationBuilder UseStatusCodePagesWithReExecute(
         var options = new StatusCodePagesOptions
         {
             HandleAsync = CreateHandler(pathFormat, queryFormat),
-            CreateScopeForErrors = createScopeForErrors
+            CreateScopeForErrors = createScopeForErrors,
+            PathFormat = pathFormat
         };
         var wrappedOptions = new OptionsWrapper<StatusCodePagesOptions>(options);
         return app.UseMiddleware<StatusCodePagesMiddleware>(wrappedOptions);
diff --git a/src/Middleware/Diagnostics/src/StatusCodePage/StatusCodePagesMiddleware.cs b/src/Middleware/Diagnostics/src/StatusCodePage/StatusCodePagesMiddleware.cs
index 87994f6c7011..7b821aa01b2e 100644
--- a/src/Middleware/Diagnostics/src/StatusCodePage/StatusCodePagesMiddleware.cs
+++ b/src/Middleware/Diagnostics/src/StatusCodePage/StatusCodePagesMiddleware.cs
@@ -48,8 +48,14 @@ public async Task Invoke(HttpContext context)
             statusCodeFeature.Enabled = false;
         }
 
+        // Attach pathFormat to HttpContext.Items early in the pipeline
+        context.Items[nameof(StatusCodePagesOptions)] = _options.PathFormat;
+
         await _next(context);
 
+        // Remove pathFormat from HttpContext.Items after handler execution
+        context.Items.Remove(nameof(StatusCodePagesOptions));
+
         if (!statusCodeFeature.Enabled)
         {
             // Check if the feature is still available because other middleware (such as a web API written in MVC) could
diff --git a/src/Middleware/Diagnostics/src/StatusCodePage/StatusCodePagesOptions.cs b/src/Middleware/Diagnostics/src/StatusCodePage/StatusCodePagesOptions.cs
index fba436b01442..95ff38eed8d5 100644
--- a/src/Middleware/Diagnostics/src/StatusCodePage/StatusCodePagesOptions.cs
+++ b/src/Middleware/Diagnostics/src/StatusCodePage/StatusCodePagesOptions.cs
@@ -62,4 +62,6 @@ private static string BuildResponseBody(int httpStatusCode)
     /// </summary>
     /// <remarks>The default value is <see langword="false"/>.</remarks>
     public bool CreateScopeForErrors { get; set; }
+
+    internal string? PathFormat { get; set; }
 }

From a586b7fa933e5f61646d37776b1b151a4c5a9979 Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Fri, 13 Jun 2025 10:36:14 +0200
Subject: [PATCH 12/28] Remove the streaming attribute.

---
 src/Components/Components/src/Routing/Router.cs | 1 -
 1 file changed, 1 deletion(-)

diff --git a/src/Components/Components/src/Routing/Router.cs b/src/Components/Components/src/Routing/Router.cs
index dbf8477a0a7f..eedff373f656 100644
--- a/src/Components/Components/src/Routing/Router.cs
+++ b/src/Components/Components/src/Routing/Router.cs
@@ -18,7 +18,6 @@ namespace Microsoft.AspNetCore.Components.Routing;
 /// <summary>
 /// A component that supplies route data corresponding to the current navigation state.
 /// </summary>
-[StreamRendering]
 public partial class Router : IComponent, IHandleAfterRender, IDisposable
 {
     // Dictionary is intentionally used instead of ReadOnlyDictionary to reduce Blazor size

From 60995b739d6b1dcae78dc3e8486c243b8812fb5c Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Fri, 13 Jun 2025 10:36:32 +0200
Subject: [PATCH 13/28] Fix the parameters order.

---
 src/Components/Web.JS/src/Rendering/StreamingRendering.ts | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/Components/Web.JS/src/Rendering/StreamingRendering.ts b/src/Components/Web.JS/src/Rendering/StreamingRendering.ts
index cefc8829f0c8..46a5a6dd9ed4 100644
--- a/src/Components/Web.JS/src/Rendering/StreamingRendering.ts
+++ b/src/Components/Web.JS/src/Rendering/StreamingRendering.ts
@@ -47,10 +47,10 @@ class BlazorStreamingUpdate extends HTMLElement {
         } else {
           switch (node.getAttribute('type')) {
             case 'redirection':
-              redirect(node, false);
+              redirect(node, true);
               break;
             case 'not-found':
-              redirect(node, true);
+              redirect(node, false);
               break;
             case 'error':
               // This is kind of brutal but matches what happens without progressive enhancement

From 96287a4f6173a0f7b0ef7cc56cd9c29570fef4cc Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Fri, 13 Jun 2025 16:40:41 +0200
Subject: [PATCH 14/28] unit test exception.

---
 .../EndpointHtmlRenderer.EventDispatch.cs     |  2 +-
 .../src/Rendering/EndpointHtmlRenderer.cs     |  5 +--
 .../test/EndpointHtmlRendererTest.cs          | 31 +++++++++++++++++++
 3 files changed, 35 insertions(+), 3 deletions(-)

diff --git a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
index 75c933521dbd..36a1074b34b5 100644
--- a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
+++ b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
@@ -79,7 +79,7 @@ private Task ReturnErrorResponse(string detailedMessage)
             : Task.CompletedTask;
     }
 
-    private async Task SetNotFoundResponseAsync(string baseUri, NotFoundEventArgs args)
+    internal async Task SetNotFoundResponseAsync(string baseUri, NotFoundEventArgs args)
     {
         if (_httpContext.Response.HasStarted)
         {
diff --git a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs
index a306680452a1..1f26984a206b 100644
--- a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs
+++ b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs
@@ -64,7 +64,7 @@ public EndpointHtmlRenderer(IServiceProvider serviceProvider, ILoggerFactory log
 
     internal HttpContext? HttpContext => _httpContext;
 
-    private void SetHttpContext(HttpContext httpContext)
+    internal void SetHttpContext(HttpContext httpContext)
     {
         if (_httpContext is null)
         {
@@ -87,7 +87,8 @@ internal async Task InitializeStandardComponentServicesAsync(
 
         if (navigationManager != null)
         {
-            navigationManager.OnNotFound += async (sender, args) => await SetNotFoundResponseAsync(navigationManager.BaseUri, args);
+            navigationManager.OnNotFound += async (sender, args) =>
+                await GetErrorHandledTask(SetNotFoundResponseAsync(navigationManager.BaseUri, args));
         }
 
         var authenticationStateProvider = httpContext.RequestServices.GetService<AuthenticationStateProvider>();
diff --git a/src/Components/Endpoints/test/EndpointHtmlRendererTest.cs b/src/Components/Endpoints/test/EndpointHtmlRendererTest.cs
index ac787b76bfe5..80e6e42e223a 100644
--- a/src/Components/Endpoints/test/EndpointHtmlRendererTest.cs
+++ b/src/Components/Endpoints/test/EndpointHtmlRendererTest.cs
@@ -1,9 +1,11 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System;
 using System.Text.Encodings.Web;
 using System.Text.Json;
 using System.Text.RegularExpressions;
+using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Components.Endpoints.Forms;
 using Microsoft.AspNetCore.Components.Endpoints.Tests.TestComponents;
 using Microsoft.AspNetCore.Components.Forms;
@@ -12,6 +14,7 @@
 using Microsoft.AspNetCore.Components.Reflection;
 using Microsoft.AspNetCore.Components.Rendering;
 using Microsoft.AspNetCore.Components.RenderTree;
+using Microsoft.AspNetCore.Components.Routing;
 using Microsoft.AspNetCore.Components.Test.Helpers;
 using Microsoft.AspNetCore.Components.Web;
 using Microsoft.AspNetCore.DataProtection;
@@ -934,6 +937,28 @@ await renderer.PrerenderComponentAsync(
         Assert.Equal("http://localhost/redirect", ctx.Response.Headers.Location);
     }
 
+    [Fact]
+    public async Task Renderer_WhenNoNotFoundPathProvided_Throws()
+    {
+        // Arrange
+        var httpContext = GetHttpContext();
+        var responseMock = new Mock<IHttpResponseFeature>();
+        responseMock.Setup(r => r.HasStarted).Returns(true);
+        responseMock.Setup(r => r.Headers).Returns(new HeaderDictionary());
+        httpContext.Features.Set(responseMock.Object);
+        var renderer = GetEndpointHtmlRenderer();
+        httpContext.Items[nameof(StatusCodePagesOptions)] = null; // simulate missing re-execution route
+
+        var component = new NotFoundComponent();
+        var id = renderer.AssignRootComponentId(component);
+        var exception = await Assert.ThrowsAsync<InvalidOperationException>(async () =>
+            await renderer.SetNotFoundResponseAsync(httpContext, new NotFoundEventArgs(""))
+        );
+        string expectedError = "The NotFoundPage route must be specified or re-execution middleware has to be set to render NotFoundPage when the response has started.";
+
+        Assert.Equal(expectedError, exception.Message);
+    }
+
     [Fact]
     public async Task CanRender_AsyncComponent()
     {
@@ -1802,6 +1827,12 @@ protected override void ProcessPendingRender()
             _rendererIsStopped = true;
             base.SignalRendererToFinishRendering();
         }
+
+        public async Task SetNotFoundResponseAsync(HttpContext httpContext, NotFoundEventArgs args)
+        {
+            SetHttpContext(httpContext);
+            await SetNotFoundResponseAsync(httpContext.Request.PathBase, args);
+        }
     }
 
     private HttpContext GetHttpContext(HttpContext context = null)

From e8aab27cb55cb66b81b825d59ea191af511086d9 Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Fri, 13 Jun 2025 17:09:58 +0200
Subject: [PATCH 15/28] Remove not used code.

---
 src/Components/Endpoints/test/EndpointHtmlRendererTest.cs | 2 --
 1 file changed, 2 deletions(-)

diff --git a/src/Components/Endpoints/test/EndpointHtmlRendererTest.cs b/src/Components/Endpoints/test/EndpointHtmlRendererTest.cs
index 80e6e42e223a..23b4b87d0b2f 100644
--- a/src/Components/Endpoints/test/EndpointHtmlRendererTest.cs
+++ b/src/Components/Endpoints/test/EndpointHtmlRendererTest.cs
@@ -949,8 +949,6 @@ public async Task Renderer_WhenNoNotFoundPathProvided_Throws()
         var renderer = GetEndpointHtmlRenderer();
         httpContext.Items[nameof(StatusCodePagesOptions)] = null; // simulate missing re-execution route
 
-        var component = new NotFoundComponent();
-        var id = renderer.AssignRootComponentId(component);
         var exception = await Assert.ThrowsAsync<InvalidOperationException>(async () =>
             await renderer.SetNotFoundResponseAsync(httpContext, new NotFoundEventArgs(""))
         );

From c6465d0d15e210d018d17b46807b0ae9540c954a Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Fri, 13 Jun 2025 18:40:30 +0200
Subject: [PATCH 16/28] Fix POST rendering without disabling the "stop render"
 signal.

---
 .../src/Rendering/EndpointHtmlRenderer.EventDispatch.cs      | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
index 36a1074b34b5..2ef85e2513b5 100644
--- a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
+++ b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
@@ -81,7 +81,10 @@ private Task ReturnErrorResponse(string detailedMessage)
 
     internal async Task SetNotFoundResponseAsync(string baseUri, NotFoundEventArgs args)
     {
-        if (_httpContext.Response.HasStarted)
+        if (_httpContext.Response.HasStarted ||
+            // POST waits for quiescence -> rendering the NotFoundPage would be queued for the next batch
+            // but we want to send the signal to the renderer to stop rendering future batches -> use client rendering
+            string.Equals(_httpContext.Request.Method, "POST", StringComparison.OrdinalIgnoreCase))
         {
             if (string.IsNullOrEmpty(_notFoundUrl))
             {

From ab0d029623bbca9b12c8925ccbf81f523fc1cf22 Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Fri, 13 Jun 2025 19:01:15 +0200
Subject: [PATCH 17/28] Unified SSR tests for POST and GET.

---
 .../NoInteractivityTest.cs                    | 175 ++++++++++++++++--
 .../ComponentThatPostsNotFound.razor          |  32 ++++
 .../testassets/Components.Shared/Index.razor  |  89 +++++++++
 .../PageThatPostsNotFound-no-streaming.razor  |  11 ++
 .../PageThatPostsNotFound-streaming.razor     |  11 ++
 .../RazorComponents/App.razor                 |   2 +-
 .../Components.WasmMinimal/Routes.razor       |   2 +-
 src/Shared/E2ETesting/WaitAssert.cs           |   5 +
 8 files changed, 305 insertions(+), 22 deletions(-)
 create mode 100644 src/Components/test/testassets/Components.Shared/ComponentThatPostsNotFound.razor
 create mode 100644 src/Components/test/testassets/Components.Shared/Index.razor
 create mode 100644 src/Components/test/testassets/Components.Shared/PageThatPostsNotFound-no-streaming.razor
 create mode 100644 src/Components/test/testassets/Components.Shared/PageThatPostsNotFound-streaming.razor

diff --git a/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs b/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
index 536eab21885a..1c1e3a2e0112 100644
--- a/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
+++ b/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
@@ -87,7 +87,7 @@ public void ProgrammaticNavigationToNotExistingPathReExecutesTo404(bool streamin
     {
         string streamingPath = streaming ? "-streaming" : "";
         Navigate($"{ServerPathBase}/reexecution/redirection-not-found-ssr{streamingPath}?navigate-programmatically=true");
-        Assert404ReExecuted();
+        AssertReExecutionPageRendered();
     }
 
     [Theory]
@@ -98,7 +98,7 @@ public void LinkNavigationToNotExistingPathReExecutesTo404(bool streaming)
         string streamingPath = streaming ? "-streaming" : "";
         Navigate($"{ServerPathBase}/reexecution/redirection-not-found-ssr{streamingPath}");
         Browser.Click(By.Id("link-to-not-existing-page"));
-        Assert404ReExecuted();
+        AssertReExecutionPageRendered();
     }
 
     [Theory]
@@ -111,40 +111,175 @@ public void BrowserNavigationToNotExistingPathReExecutesTo404(bool streaming)
         // will not be activated, see configuration in Startup
         string streamingPath = streaming ? "-streaming" : "";
         Navigate($"{ServerPathBase}/reexecution/not-existing-page-ssr{streamingPath}");
-        Assert404ReExecuted();
+        AssertReExecutionPageRendered();
     }
 
-    private void Assert404ReExecuted() =>
+    private void AssertReExecutionPageRendered() =>
         Browser.Equal("Welcome On Page Re-executed After Not Found Event", () => Browser.Exists(By.Id("test-info")).Text);
 
+    private void AssertNotFoundPageRendered()
+    {
+        Browser.Equal("Welcome On Custom Not Found Page", () => Browser.FindElement(By.Id("test-info")).Text);
+        // custom page should have a custom layout
+        Browser.Equal("About", () => Browser.FindElement(By.Id("about-link")).Text);
+    }
+
+    private void AssertUrlNotChanged(string expectedUrl) =>
+        Browser.True(() => Browser.Url.Contains(expectedUrl), $"Expected URL to contain '{expectedUrl}', but found '{Browser.Url}'");
+
+    private void AssertUrlChanged(string urlPart) =>
+        Browser.False(() => Browser.Url.Contains(urlPart), $"Expected URL not to contain '{urlPart}', but found '{Browser.Url}'");
+
     [Theory]
-    [InlineData(true)]
-    [InlineData(false)]
-    public void CanRenderNotFoundPage(bool streamingStarted)
+    [InlineData(true, true)]
+    [InlineData(true, false)]
+    [InlineData(false, true)]
+    [InlineData(false, false)]
+    // When response has not started, it does not matter how we arrive the the page setting the not found status code.
+    // We can navigate straight to the testing page, skipping the index page.
+    public void NotFoundSetOnInitialization_ResponseNotStarted_SSR(bool hasReExecutionMiddleware, bool hasCustomNotFoundPageSet)
     {
-        string streamingPath = streamingStarted ? "-streaming" : "";
-        Navigate($"{ServerPathBase}/set-not-found-ssr{streamingPath}?useCustomNotFoundPage=true");
+        string reexecution = hasReExecutionMiddleware ? "/reexecution" : "";
+        string testUrl = $"{ServerPathBase}{reexecution}/set-not-found-ssr?useCustomNotFoundPage={hasCustomNotFoundPageSet}";
+        Navigate(testUrl);
 
-        var infoText = Browser.FindElement(By.Id("test-info")).Text;
-        Assert.Contains("Welcome On Custom Not Found Page", infoText);
-        // custom page should have a custom layout
-        var aboutLink = Browser.FindElement(By.Id("about-link")).Text;
-        Assert.Contains("About", aboutLink);
+        if (hasCustomNotFoundPageSet)
+        {
+            AssertNotFoundPageRendered();
+        }
+        else
+        {
+            AssertNotFoundFragmentRendered();
+        }
+        AssertUrlNotChanged(testUrl);
     }
 
+    // If the response has started, it matters how we arrive the the page setting the not found status code
+    // because we are rendering client-side. In browser-initiated navigation and form submissions, the headers
+    // don't contain enhanced-nav and we redirect to the not found page. In link clicking navigation,
+    // the headers contain enhanced-nav, redirection is not needed and the original url will be preserved.
+
     [Theory]
-    [InlineData(false)]
-    [InlineData(true)]
-    public void DoesNotReExecuteIf404WasHandled(bool streamingStarted)
+    [InlineData(true, true)] // manual testing works, why Selenium does not?
+    [InlineData(true, false)]
+    [InlineData(false, true)] // manual testing works, why Selenium does not?
+    [InlineData(false, false)]
+    // enhanced navigation is switched off for browser navigation
+    public void NotFoundSetOnInitialization_ResponseStarted_BrowserNavigation_SSR(bool hasReExecutionMiddleware, bool hasCustomNotFoundPageSet)
+    {
+        string reexecution = hasReExecutionMiddleware ? "/reexecution" : "";
+        string testUrl = $"{ServerPathBase}{reexecution}/set-not-found-ssr-streaming?useCustomNotFoundPage={hasCustomNotFoundPageSet}";
+        Navigate(testUrl);
+        AssertNotFoundRendered_ResponseStarted_Or_POST(hasReExecutionMiddleware, hasCustomNotFoundPageSet, testUrl);
+        bool throwsException = !hasCustomNotFoundPageSet && !hasReExecutionMiddleware;
+        if (throwsException)
+        {
+            AssertUrlNotChanged(testUrl);
+        }
+        else
+        {
+            AssertUrlChanged(testUrl);
+        }
+    }
+
+    private void AssertNotFoundRendered_ResponseStarted_Or_POST(bool hasReExecutionMiddleware, bool hasCustomNotFoundPageSet, string testUrl)
+    {
+        if (hasCustomNotFoundPageSet)
+        {
+            AssertNotFoundPageRendered();
+        }
+        else if (hasReExecutionMiddleware)
+        {
+            AssertReExecutionPageRendered();
+        }
+        else
+        {
+            // this throws an exception logged the server
+            AssertNotFoundContentNotRendered();
+        }
+    }
+
+    [Theory]
+    //[InlineData(true, true)] // ActiveIssue("NotFoundPageRoute is not set in Router")
+    [InlineData(true, false)]
+    //[InlineData(false, true)] // ActiveIssue("NotFoundPageRoute is not set in Router")
+    [InlineData(false, false)]
+    // enhanced navigation is switched on for link navigation
+    public void NotFoundSetOnInitialization_ResponseStarted_LinkNavigation_SSR(bool hasReExecutionMiddleware, bool hasCustomNotFoundPageSet)
+    {
+        string testUrl = NavigateByLinkToPageTestingNotFound("Sets", hasReExecutionMiddleware, hasCustomNotFoundPageSet);
+        AssertNotFoundRendered_ResponseStarted_Or_POST(hasReExecutionMiddleware, hasCustomNotFoundPageSet, testUrl);
+        AssertUrlNotChanged(testUrl);
+    }
+
+    private string NavigateByLinkToPageTestingNotFound(string action, bool hasReExecutionMiddleware, bool hasCustomNotFoundPageSet)
+    {
+        string reexecution = hasReExecutionMiddleware ? "/reexecution" : "";
+        Navigate($"{ServerPathBase}/not-found-index?useCustomNotFoundPage={hasCustomNotFoundPageSet}");
+        Browser.Equal("List of Not Found test pages", () => Browser.FindElement(By.Id("test-info")).Text);
+        string reexecutionText = hasReExecutionMiddleware ? " with reexecution" : "";
+        var link = Browser.FindElement(By.LinkText($"PageThat{action}NotFound-streaming{reexecutionText}"));
+        var testUrl = link.GetAttribute("href");
+        link.Click();
+        return testUrl;
+    }
+
+    [Theory]
+    //[InlineData(true, true)] // ActiveIssue("NotFoundPageRoute is not set in Router")
+    [InlineData(true, false)]
+    //[InlineData(false, true)] // ActiveIssue("NotFoundPageRoute is not set in Router")
+    [InlineData(false, false)]
+    // NotFound triggered by POST cannot get rendered in the same batch and we rely on the client to render it
+    // However, because it is triggered by form POST, it won't have enhanced-nav headers
+    public void NotFoundSetOnFormSubmit_ResponseNotStarted_SSR(bool hasReExecutionMiddleware, bool hasCustomNotFoundPageSet)
+    {
+        string reexecution = hasReExecutionMiddleware ? "/reexecution" : "";
+        string testUrl = $"{ServerPathBase}{reexecution}/post-not-found-ssr-streaming?useCustomNotFoundPage={hasCustomNotFoundPageSet}";
+        Navigate(testUrl);
+        Browser.FindElement(By.Id("not-found-form")).FindElement(By.TagName("button")).Click();
+
+        AssertNotFoundRendered_ResponseStarted_Or_POST(hasReExecutionMiddleware, hasCustomNotFoundPageSet, testUrl);
+        bool throwsException = !hasCustomNotFoundPageSet && !hasReExecutionMiddleware;
+        if (throwsException)
+        {
+            AssertUrlNotChanged(testUrl);
+        }
+        else
+        {
+            AssertUrlChanged(testUrl);
+        }
+    }
+
+    [Theory]
+    //[InlineData(true, true)] // ActiveIssue("NotFoundPageRoute is not set in Router")
+    [InlineData(true, false)]
+    //[InlineData(false, true)] // ActiveIssue("NotFoundPageRoute is not set in Router")
+    [InlineData(false, false)]
+    public void NotFoundSetOnFormSubmit_ResponseStarted_SSR(bool hasReExecutionMiddleware, bool hasCustomNotFoundPageSet)
     {
-        string streamingPath = streamingStarted ? "-streaming" : "";
-        Navigate($"{ServerPathBase}/reexecution/set-not-found-ssr{streamingPath}");
-        AssertNotFoundFragmentRendered();
+        string reexecution = hasReExecutionMiddleware ? "/reexecution" : "";
+        string testUrl = $"{ServerPathBase}{reexecution}/post-not-found-ssr-streaming?useCustomNotFoundPage={hasCustomNotFoundPageSet}";
+        Navigate(testUrl);
+        Browser.FindElement(By.Id("not-found-form")).FindElement(By.TagName("button")).Click();
+
+        AssertNotFoundRendered_ResponseStarted_Or_POST(hasReExecutionMiddleware, hasCustomNotFoundPageSet, testUrl);
+        bool throwsException = !hasCustomNotFoundPageSet && !hasReExecutionMiddleware;
+        if (throwsException)
+        {
+            AssertUrlNotChanged(testUrl);
+        }
+        else
+        {
+            AssertUrlChanged(testUrl);
+        }
     }
 
     private void AssertNotFoundFragmentRendered() =>
         Browser.Equal("There's nothing here", () => Browser.FindElement(By.Id("not-found-fragment")).Text);
 
+    private void AssertNotFoundContentNotRendered() =>
+        Browser.Equal("Any content", () => Browser.FindElement(By.Id("test-info")).Text);
+
     [Fact]
     public void StatusCodePagesWithReExecution()
     {
diff --git a/src/Components/test/testassets/Components.Shared/ComponentThatPostsNotFound.razor b/src/Components/test/testassets/Components.Shared/ComponentThatPostsNotFound.razor
new file mode 100644
index 000000000000..f9b6cd9c7a3c
--- /dev/null
+++ b/src/Components/test/testassets/Components.Shared/ComponentThatPostsNotFound.razor
@@ -0,0 +1,32 @@
+@using Microsoft.AspNetCore.Components.Forms
+
+@inject NavigationManager NavigationManager
+
+@if (!WaitForInteractivity || RendererInfo.IsInteractive)
+{
+    <PageTitle>Original page</PageTitle>
+
+    <form method="post" id="not-found-form" @onsubmit="HandleSubmit" @formname="PostNotFoundForm">
+        <AntiforgeryToken />
+        <button type="submit">Trigger NotFound</button>
+    </form>
+
+    <p id="test-info">Any content</p>
+}
+
+@code{
+    [Parameter]
+    public bool StartStreaming { get; set; } = false;
+
+    [Parameter]
+    public bool WaitForInteractivity { get; set; } = false;
+
+    private async Task HandleSubmit()
+    {
+        if (StartStreaming)
+        {
+            await Task.Yield();
+        }
+        NavigationManager.NotFound();
+    }
+}
\ No newline at end of file
diff --git a/src/Components/test/testassets/Components.Shared/Index.razor b/src/Components/test/testassets/Components.Shared/Index.razor
new file mode 100644
index 000000000000..d99fff1fa192
--- /dev/null
+++ b/src/Components/test/testassets/Components.Shared/Index.razor
@@ -0,0 +1,89 @@
+@page "/not-found-index"
+
+@using Microsoft.AspNetCore.Components.Forms
+
+@inject NavigationManager NavigationManager
+
+<h1 id="test-info">List of Not Found test pages</h1>
+
+<h2>Link navigation (enhanced nav passed in Headers)</h2>
+<ul>
+    <li><a href="@PostNotFoundNoStreaming">@PostNotFoundNoStreamingText</a></li>
+    <li><a href="@PostNotFoundNoStreamingReexecution">@PostNotFoundNoStreamingReexecutionText</a></li>
+
+    <li><a href="@PostNotFoundStreaming">@PostNotFoundStreamingText</a></li>
+    <li><a href="@PostNotFoundStreamingReexecution">@PostNotFoundStreamingReexecutionText</a></li>
+
+    <li><a href="@SetNotFoundStreaming">@SetNotFoundNoStreamingText</a></li>
+    <li><a href="@SetNotFoundNoStreamingReexecution">@SetNotFoundNoStreamingReexecutionText</a></li>
+
+    <li><a href="@SetNotFoundStreaming">@SetNotFoundStreamingText</a></li>
+    <li><a href="@SetNotFoundStreamingReexecution">@SetNotFoundStreamingReexecutionText</a></li>
+</ul>
+
+<h2>Programmatic navigation in a form (enhanced nav not passed in Headers)</h2>
+<form method="post" @onsubmit="() => NavigateTo(PostNotFoundNoStreaming)" @formname="ProgrammaticPostNotFoundNoStreamingForm">
+    <AntiforgeryToken />
+    <button type="submit">Programmatic navigate to @PostNotFoundNoStreamingText</button>
+</form>
+<form method="post" @onsubmit="() => NavigateTo(PostNotFoundNoStreamingReexecution)" @formname="ProgrammaticPostNotFoundNoStreamingReexecutionForm">
+    <AntiforgeryToken />
+    <button type="submit">Programmatic navigate to @PostNotFoundNoStreamingReexecutionText</button>
+</form>
+
+<form method="post" @onsubmit="() => NavigateTo(PostNotFoundStreaming)" @formname="ProgrammaticPostNotFoundStreamingForm">
+    <AntiforgeryToken />
+    <button type="submit">Programmatic navigate to @PostNotFoundStreamingText</button>
+</form>
+<form method="post" @onsubmit="() => NavigateTo(PostNotFoundStreamingReexecution)" @formname="ProgrammaticPostNotFoundStreamingReexecutionForm">
+    <AntiforgeryToken />
+    <button type="submit">Programmatic navigate to @PostNotFoundStreamingReexecutionText</button>
+</form>
+
+<form method="post" @onsubmit="() => NavigateTo(SetNotFoundNoStreaming)" @formname="ProgrammaticSetNotFoundNoStreamingForm">
+    <AntiforgeryToken />
+    <button type="submit">Programmatic navigate to @SetNotFoundNoStreamingText</button>
+</form>
+<form method="post" @onsubmit="() => NavigateTo(SetNotFoundNoStreamingReexecution)" @formname="ProgrammaticSetNotFoundNoStreamingReexecutionForm">
+    <AntiforgeryToken />
+    <button type="submit">Programmatic navigate to @SetNotFoundNoStreamingReexecutionText</button>
+</form>
+
+<form method="post" @onsubmit="() => NavigateTo(SetNotFoundStreaming)" @formname="ProgrammaticSetNotFoundStreamingForm">
+    <AntiforgeryToken />
+    <button type="submit">Programmatic navigate to @SetNotFoundStreamingText</button>
+</form>
+<form method="post" @onsubmit="() => NavigateTo(SetNotFoundStreamingReexecution)" @formname="ProgrammaticSetNotFoundStreamingReexecutionForm">
+    <AntiforgeryToken />
+    <button type="submit">Programmatic navigate to @SetNotFoundStreamingReexecutionText</button>
+</form>
+
+@code {
+    [SupplyParameterFromQuery]
+    public bool useCustomNotFoundPage { get; set; }
+
+    private string AppendQuery(string url) => $"{url}?useCustomNotFoundPage={useCustomNotFoundPage}";
+
+    private string PostNotFoundNoStreaming => AppendQuery("post-not-found-ssr");
+    private string PostNotFoundNoStreamingReexecution => AppendQuery($"reexecution/post-not-found-ssr");
+    private string PostNotFoundStreaming => AppendQuery("post-not-found-ssr-streaming");
+    private string PostNotFoundStreamingReexecution => AppendQuery($"reexecution/post-not-found-ssr-streaming");
+    private string SetNotFoundNoStreaming => AppendQuery("set-not-found-ssr");
+    private string SetNotFoundNoStreamingReexecution => AppendQuery($"reexecution/set-not-found-ssr");
+    private string SetNotFoundStreaming => AppendQuery("set-not-found-ssr-streaming");
+    private string SetNotFoundStreamingReexecution => AppendQuery($"reexecution/set-not-found-ssr-streaming");
+
+    private const string PostNotFoundNoStreamingText = "PageThatPostsNotFound-no-streaming";
+    private const string PostNotFoundNoStreamingReexecutionText = $"{PostNotFoundNoStreamingText} with reexecution";
+    private const string PostNotFoundStreamingText = "PageThatPostsNotFound-streaming";
+    private const string PostNotFoundStreamingReexecutionText = $"{PostNotFoundStreamingText} with reexecution";
+    private const string SetNotFoundNoStreamingText = "PageThatSetsNotFound-no-streaming";
+    private const string SetNotFoundNoStreamingReexecutionText = $"{SetNotFoundNoStreamingText} with reexecution";
+    private const string SetNotFoundStreamingText = "PageThatSetsNotFound-streaming";
+    private const string SetNotFoundStreamingReexecutionText = $"{SetNotFoundStreamingText} with reexecution";
+
+    private void NavigateTo(string url)
+    {
+        NavigationManager.NavigateTo(url);
+    }
+}
\ No newline at end of file
diff --git a/src/Components/test/testassets/Components.Shared/PageThatPostsNotFound-no-streaming.razor b/src/Components/test/testassets/Components.Shared/PageThatPostsNotFound-no-streaming.razor
new file mode 100644
index 000000000000..d02932a8c4e5
--- /dev/null
+++ b/src/Components/test/testassets/Components.Shared/PageThatPostsNotFound-no-streaming.razor
@@ -0,0 +1,11 @@
+@page "/reexecution/post-not-found-ssr"
+@page "/post-not-found-ssr"
+@attribute [StreamRendering(false)]
+
+@*
+    this page is used in global interactivity and no interactivity scenarios
+    the content is rendered on the server without streaming and might become
+    interactive later if interactivity was enabled in the app
+*@
+
+<Components.Shared.ComponentThatPostsNotFound />
\ No newline at end of file
diff --git a/src/Components/test/testassets/Components.Shared/PageThatPostsNotFound-streaming.razor b/src/Components/test/testassets/Components.Shared/PageThatPostsNotFound-streaming.razor
new file mode 100644
index 000000000000..93ac9fc23359
--- /dev/null
+++ b/src/Components/test/testassets/Components.Shared/PageThatPostsNotFound-streaming.razor
@@ -0,0 +1,11 @@
+@page "/reexecution/post-not-found-ssr-streaming"
+@page "/post-not-found-ssr-streaming"
+@attribute [StreamRendering(true)]
+
+@*
+    this page is used in global interactivity and no interactivity scenarios
+    the content is rendered on the server with streaming and might become
+    interactive later if interactivity was enabled in the app
+*@
+
+<Components.Shared.ComponentThatPostsNotFound StartStreaming="true"/>
\ No newline at end of file
diff --git a/src/Components/test/testassets/Components.TestServer/RazorComponents/App.razor b/src/Components/test/testassets/Components.TestServer/RazorComponents/App.razor
index 9912b1055e79..3ada2c590153 100644
--- a/src/Components/test/testassets/Components.TestServer/RazorComponents/App.razor
+++ b/src/Components/test/testassets/Components.TestServer/RazorComponents/App.razor
@@ -10,7 +10,7 @@
 
     protected override void OnParametersSet()
     {
-        if (UseCustomNotFoundPage == "true")
+        if (string.Equals(UseCustomNotFoundPage, "true", StringComparison.OrdinalIgnoreCase))
         {
             NotFoundPageType = typeof(CustomNotFoundPage);
         }
diff --git a/src/Components/test/testassets/Components.WasmMinimal/Routes.razor b/src/Components/test/testassets/Components.WasmMinimal/Routes.razor
index 18226373e31c..d8ee0e62498c 100644
--- a/src/Components/test/testassets/Components.WasmMinimal/Routes.razor
+++ b/src/Components/test/testassets/Components.WasmMinimal/Routes.razor
@@ -11,7 +11,7 @@
 
     protected override void OnParametersSet()
     {
-        if (UseCustomNotFoundPage == "true")
+        if (string.Equals(UseCustomNotFoundPage, "true", StringComparison.OrdinalIgnoreCase))
         {
             NotFoundPageType = typeof(CustomNotFoundPage);
         }
diff --git a/src/Shared/E2ETesting/WaitAssert.cs b/src/Shared/E2ETesting/WaitAssert.cs
index 71c8ae6b7c41..a3bb53520f30 100644
--- a/src/Shared/E2ETesting/WaitAssert.cs
+++ b/src/Shared/E2ETesting/WaitAssert.cs
@@ -37,6 +37,11 @@ public static void True(this IWebDriver driver, Func<bool> actual, TimeSpan time
 
     public static void True(this IWebDriver driver, Func<bool> actual, TimeSpan timeout, string message)
         => WaitAssertCore(driver, () => Assert.True(actual(), message), timeout);
+    public static void True(this IWebDriver driver, Func<bool> actual, string message)
+        => WaitAssertCore(driver, () => Assert.True(actual(), message));
+
+    public static void False(this IWebDriver driver, Func<bool> actual, string message)
+        => WaitAssertCore(driver, () => Assert.False(actual(), message));
 
     public static void False(this IWebDriver driver, Func<bool> actual)
         => WaitAssertCore(driver, () => Assert.False(actual()));

From 3d20fa9089dee0c26ae3546c62672231817cfe89 Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Fri, 13 Jun 2025 19:40:00 +0200
Subject: [PATCH 18/28] Remove comments, test on CI

---
 .../NoInteractivityTest.cs                     | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs b/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
index 1c1e3a2e0112..c27997b7710f 100644
--- a/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
+++ b/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
@@ -160,9 +160,9 @@ public void NotFoundSetOnInitialization_ResponseNotStarted_SSR(bool hasReExecuti
     // the headers contain enhanced-nav, redirection is not needed and the original url will be preserved.
 
     [Theory]
-    [InlineData(true, true)] // manual testing works, why Selenium does not?
+    [InlineData(true, true)]
     [InlineData(true, false)]
-    [InlineData(false, true)] // manual testing works, why Selenium does not?
+    [InlineData(false, true)]
     [InlineData(false, false)]
     // enhanced navigation is switched off for browser navigation
     public void NotFoundSetOnInitialization_ResponseStarted_BrowserNavigation_SSR(bool hasReExecutionMiddleware, bool hasCustomNotFoundPageSet)
@@ -194,15 +194,15 @@ private void AssertNotFoundRendered_ResponseStarted_Or_POST(bool hasReExecutionM
         }
         else
         {
-            // this throws an exception logged the server
+            // this throws an exception logged on the server
             AssertNotFoundContentNotRendered();
         }
     }
 
     [Theory]
-    //[InlineData(true, true)] // ActiveIssue("NotFoundPageRoute is not set in Router")
+    [InlineData(true, true)]
     [InlineData(true, false)]
-    //[InlineData(false, true)] // ActiveIssue("NotFoundPageRoute is not set in Router")
+    [InlineData(false, true)]
     [InlineData(false, false)]
     // enhanced navigation is switched on for link navigation
     public void NotFoundSetOnInitialization_ResponseStarted_LinkNavigation_SSR(bool hasReExecutionMiddleware, bool hasCustomNotFoundPageSet)
@@ -225,9 +225,9 @@ private string NavigateByLinkToPageTestingNotFound(string action, bool hasReExec
     }
 
     [Theory]
-    //[InlineData(true, true)] // ActiveIssue("NotFoundPageRoute is not set in Router")
+    [InlineData(true, true)]
     [InlineData(true, false)]
-    //[InlineData(false, true)] // ActiveIssue("NotFoundPageRoute is not set in Router")
+    [InlineData(false, true)]
     [InlineData(false, false)]
     // NotFound triggered by POST cannot get rendered in the same batch and we rely on the client to render it
     // However, because it is triggered by form POST, it won't have enhanced-nav headers
@@ -251,9 +251,9 @@ public void NotFoundSetOnFormSubmit_ResponseNotStarted_SSR(bool hasReExecutionMi
     }
 
     [Theory]
-    //[InlineData(true, true)] // ActiveIssue("NotFoundPageRoute is not set in Router")
+    [InlineData(true, true)]
     [InlineData(true, false)]
-    //[InlineData(false, true)] // ActiveIssue("NotFoundPageRoute is not set in Router")
+    [InlineData(false, true)]
     [InlineData(false, false)]
     public void NotFoundSetOnFormSubmit_ResponseStarted_SSR(bool hasReExecutionMiddleware, bool hasCustomNotFoundPageSet)
     {

From 1caeaa6dec70fb5f42b85aaa6503f7108f4474b4 Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Fri, 13 Jun 2025 19:43:25 +0200
Subject: [PATCH 19/28] Revert unnecessary changes.

---
 .../Mvc.FunctionalTests/ComponentRenderingFunctionalTests.cs    | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/Mvc/test/Mvc.FunctionalTests/ComponentRenderingFunctionalTests.cs b/src/Mvc/test/Mvc.FunctionalTests/ComponentRenderingFunctionalTests.cs
index 11ed2cd95595..d319827f9a33 100644
--- a/src/Mvc/test/Mvc.FunctionalTests/ComponentRenderingFunctionalTests.cs
+++ b/src/Mvc/test/Mvc.FunctionalTests/ComponentRenderingFunctionalTests.cs
@@ -89,7 +89,7 @@ public async Task Renders_RoutingComponent_UsingRazorComponents_Prerenderer()
         await response.AssertStatusCodeAsync(HttpStatusCode.OK);
         var content = await response.Content.ReadAsStringAsync();
 
-        AssertComponent("Router component\n<!--bl:3--><!--bl:4--><!--bl:5--><!--bl:6--><p>Routed successfully</p><!--/bl:6--><!--/bl:5--><!--/bl:4--><!--/bl:3-->", "Routing", content);
+        AssertComponent("Router component\n<p>Routed successfully</p>", "Routing", content);
     }
 
     [Fact]

From fed0e5aaa375f7a5a5fcbac9f372ccc0fc1954af Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Fri, 13 Jun 2025 20:34:54 +0200
Subject: [PATCH 20/28] Clean up global interactivity tests.

---
 .../E2ETest/Tests/GlobalInteractivityTest.cs  | 35 ++++++++++---------
 1 file changed, 18 insertions(+), 17 deletions(-)

diff --git a/src/Components/test/E2ETest/Tests/GlobalInteractivityTest.cs b/src/Components/test/E2ETest/Tests/GlobalInteractivityTest.cs
index 368cfab8251c..7a1163de31c8 100644
--- a/src/Components/test/E2ETest/Tests/GlobalInteractivityTest.cs
+++ b/src/Components/test/E2ETest/Tests/GlobalInteractivityTest.cs
@@ -37,7 +37,7 @@ public void CanRenderNotFoundInteractive(string renderingMode, bool useCustomNot
 
         if (useCustomNotFoundPage)
         {
-            AssertCustomNotFoundPageRendered();
+            AssertNotFoundPageRendered();
         }
         else
         {
@@ -144,7 +144,7 @@ public void CanRenderNotFoundPage_SSR(bool streamingStarted)
     {
         string streamingPath = streamingStarted ? "-streaming" : "";
         Navigate($"{ServerPathBase}/set-not-found-ssr{streamingPath}?useCustomNotFoundPage=true");
-        AssertCustomNotFoundPageRendered();
+        AssertNotFoundPageRendered();
     }
 
     [Theory]
@@ -153,26 +153,31 @@ public void CanRenderNotFoundPage_SSR(bool streamingStarted)
     public void CanRenderNotFoundPage_Interactive(string renderMode)
     {
         Navigate($"{ServerPathBase}/set-not-found?useCustomNotFoundPage=true&renderMode={renderMode}");
-        AssertCustomNotFoundPageRendered();
+        AssertNotFoundPageRendered();
     }
 
-    private void AssertCustomNotFoundPageRendered()
+    private void AssertNotFoundPageRendered()
     {
-        var infoText = Browser.FindElement(By.Id("test-info")).Text;
-        Assert.Contains("Welcome On Custom Not Found Page", infoText);
+        Browser.Equal("Welcome On Custom Not Found Page", () => Browser.FindElement(By.Id("test-info")).Text);
         // custom page should have a custom layout
-        var aboutLink = Browser.FindElement(By.Id("about-link")).Text;
-        Assert.Contains("About", aboutLink);
+        Browser.Equal("About", () => Browser.FindElement(By.Id("about-link")).Text);
     }
 
     [Theory]
     [InlineData(false)]
     [InlineData(true)]
-    public void DoesNotReExecuteIf404WasHandled_SSR(bool streamingStarted)
+    public void CanRenderNotFoundIfNotFoundPageTypeNotProvided_SSR(bool streamingStarted)
     {
         string streamingPath = streamingStarted ? "-streaming" : "";
         Navigate($"{ServerPathBase}/reexecution/set-not-found-ssr{streamingPath}");
-        AssertNotFoundFragmentRendered();
+        if (streamingStarted)
+        {
+            AssertReExecutedPageRendered();
+        }
+        else
+        {
+            AssertNotFoundFragmentRendered();
+        }
     }
 
     [Theory]
@@ -184,19 +189,15 @@ public void DoesNotReExecuteIf404WasHandled_Interactive(string renderMode)
         AssertNotFoundFragmentRendered();
     }
 
-    private void AssertNotFoundFragmentRendered()
-    {
-        var body = Browser.FindElement(By.TagName("body"));
-        var notFound = Browser.FindElement(By.Id("not-found-fragment")).Text;
+    private void AssertNotFoundFragmentRendered() =>
         Browser.Equal("There's nothing here", () => Browser.FindElement(By.Id("not-found-fragment")).Text);
-    }
 
     [Fact]
     public void StatusCodePagesWithReExecution()
     {
         Navigate($"{ServerPathBase}/reexecution/trigger-404");
-        Assert404ReExecuted();
+        AssertReExecutedPageRendered();
     }
-    private void Assert404ReExecuted() =>
+    private void AssertReExecutedPageRendered() =>
         Browser.Equal("Welcome On Page Re-executed After Not Found Event", () => Browser.Exists(By.Id("test-info")).Text);
 }

From 1c6f97da2f6add3ff3161802bdc54e991af75ab4 Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Fri, 13 Jun 2025 21:05:14 +0200
Subject: [PATCH 21/28] Trying to fix "Failed to load resource"

---
 .../Pages/NotFound/CustomNotFoundPage.razor                      | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/Components/test/testassets/Components.WasmMinimal/Pages/NotFound/CustomNotFoundPage.razor b/src/Components/test/testassets/Components.WasmMinimal/Pages/NotFound/CustomNotFoundPage.razor
index ed03f17c7ccb..1a744d504811 100644
--- a/src/Components/test/testassets/Components.WasmMinimal/Pages/NotFound/CustomNotFoundPage.razor
+++ b/src/Components/test/testassets/Components.WasmMinimal/Pages/NotFound/CustomNotFoundPage.razor
@@ -1,4 +1,5 @@
 @page "/render-custom-not-found-page"
+@page "/reexecution/render-custom-not-found-page"
 @layout NotFoundLayout
 
 <h3 id="test-info">Welcome On Custom Not Found Page</h3>

From 9d52115eb9c2ac792ba7672dc75e55f610d93d48 Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Fri, 13 Jun 2025 21:17:01 +0200
Subject: [PATCH 22/28] Fix tests with custom not found page.

---
 .../test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs    | 2 +-
 .../NotFound => Components.Shared}/CustomNotFoundPage.razor     | 1 -
 .../Pages/NotFound => Components.Shared}/NotFoundLayout.razor   | 0
 .../testassets/Components.TestServer/RazorComponents/App.razor  | 1 +
 .../test/testassets/Components.WasmMinimal/Routes.razor         | 1 +
 5 files changed, 3 insertions(+), 2 deletions(-)
 rename src/Components/test/testassets/{Components.WasmMinimal/Pages/NotFound => Components.Shared}/CustomNotFoundPage.razor (78%)
 rename src/Components/test/testassets/{Components.WasmMinimal/Pages/NotFound => Components.Shared}/NotFoundLayout.razor (100%)

diff --git a/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs b/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
index c27997b7710f..760db83fb3eb 100644
--- a/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
+++ b/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
@@ -157,7 +157,7 @@ public void NotFoundSetOnInitialization_ResponseNotStarted_SSR(bool hasReExecuti
     // If the response has started, it matters how we arrive the the page setting the not found status code
     // because we are rendering client-side. In browser-initiated navigation and form submissions, the headers
     // don't contain enhanced-nav and we redirect to the not found page. In link clicking navigation,
-    // the headers contain enhanced-nav, redirection is not needed and the original url will be preserved.
+    // the headers contain enhanced-nav, redirection is not needed and the original url is preserved.
 
     [Theory]
     [InlineData(true, true)]
diff --git a/src/Components/test/testassets/Components.WasmMinimal/Pages/NotFound/CustomNotFoundPage.razor b/src/Components/test/testassets/Components.Shared/CustomNotFoundPage.razor
similarity index 78%
rename from src/Components/test/testassets/Components.WasmMinimal/Pages/NotFound/CustomNotFoundPage.razor
rename to src/Components/test/testassets/Components.Shared/CustomNotFoundPage.razor
index 1a744d504811..ed03f17c7ccb 100644
--- a/src/Components/test/testassets/Components.WasmMinimal/Pages/NotFound/CustomNotFoundPage.razor
+++ b/src/Components/test/testassets/Components.Shared/CustomNotFoundPage.razor
@@ -1,5 +1,4 @@
 @page "/render-custom-not-found-page"
-@page "/reexecution/render-custom-not-found-page"
 @layout NotFoundLayout
 
 <h3 id="test-info">Welcome On Custom Not Found Page</h3>
diff --git a/src/Components/test/testassets/Components.WasmMinimal/Pages/NotFound/NotFoundLayout.razor b/src/Components/test/testassets/Components.Shared/NotFoundLayout.razor
similarity index 100%
rename from src/Components/test/testassets/Components.WasmMinimal/Pages/NotFound/NotFoundLayout.razor
rename to src/Components/test/testassets/Components.Shared/NotFoundLayout.razor
diff --git a/src/Components/test/testassets/Components.TestServer/RazorComponents/App.razor b/src/Components/test/testassets/Components.TestServer/RazorComponents/App.razor
index 3ada2c590153..dc696f1a0f2d 100644
--- a/src/Components/test/testassets/Components.TestServer/RazorComponents/App.razor
+++ b/src/Components/test/testassets/Components.TestServer/RazorComponents/App.razor
@@ -1,5 +1,6 @@
 @using Components.TestServer.RazorComponents.Pages.Forms
 @using Components.WasmMinimal.Pages.NotFound
+@using Components.Shared
 
 @code {
     [Parameter]
diff --git a/src/Components/test/testassets/Components.WasmMinimal/Routes.razor b/src/Components/test/testassets/Components.WasmMinimal/Routes.razor
index d8ee0e62498c..5a2ba90aaed4 100644
--- a/src/Components/test/testassets/Components.WasmMinimal/Routes.razor
+++ b/src/Components/test/testassets/Components.WasmMinimal/Routes.razor
@@ -1,5 +1,6 @@
 @using Microsoft.AspNetCore.Components.Routing
 @using Components.WasmMinimal.Pages.NotFound
+@using Components.Shared
 @inject NavigationManager NavigationManager
 
 @code {

From 41a186c203703b62e2b5d6994425d7c461ce9a3d Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <32700855+ilonatommy@users.noreply.github.com>
Date: Mon, 16 Jun 2025 09:31:50 +0200
Subject: [PATCH 23/28] Update
 src/Components/test/testassets/Components.Shared/Index.razor

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
---
 src/Components/test/testassets/Components.Shared/Index.razor | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/Components/test/testassets/Components.Shared/Index.razor b/src/Components/test/testassets/Components.Shared/Index.razor
index d99fff1fa192..5d0bd8b17ddc 100644
--- a/src/Components/test/testassets/Components.Shared/Index.razor
+++ b/src/Components/test/testassets/Components.Shared/Index.razor
@@ -14,7 +14,7 @@
     <li><a href="@PostNotFoundStreaming">@PostNotFoundStreamingText</a></li>
     <li><a href="@PostNotFoundStreamingReexecution">@PostNotFoundStreamingReexecutionText</a></li>
 
-    <li><a href="@SetNotFoundStreaming">@SetNotFoundNoStreamingText</a></li>
+    <li><a href="@SetNotFoundNoStreaming">@SetNotFoundNoStreamingText</a></li>
     <li><a href="@SetNotFoundNoStreamingReexecution">@SetNotFoundNoStreamingReexecutionText</a></li>
 
     <li><a href="@SetNotFoundStreaming">@SetNotFoundStreamingText</a></li>

From f7c609207313cae659cbd1732a8c97a0c17e8778 Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Mon, 16 Jun 2025 13:24:38 +0200
Subject: [PATCH 24/28] Feedback: neat POST detection and redundant awaits
 removal.

---
 .../src/Rendering/EndpointHtmlRenderer.EventDispatch.cs    | 2 +-
 .../src/Rendering/EndpointHtmlRenderer.Streaming.cs        | 2 +-
 .../Endpoints/src/Rendering/EndpointHtmlRenderer.cs        | 7 +++----
 3 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
index 2ef85e2513b5..cdf17e376a00 100644
--- a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
+++ b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs
@@ -84,7 +84,7 @@ internal async Task SetNotFoundResponseAsync(string baseUri, NotFoundEventArgs a
         if (_httpContext.Response.HasStarted ||
             // POST waits for quiescence -> rendering the NotFoundPage would be queued for the next batch
             // but we want to send the signal to the renderer to stop rendering future batches -> use client rendering
-            string.Equals(_httpContext.Request.Method, "POST", StringComparison.OrdinalIgnoreCase))
+            HttpMethods.IsPost(_httpContext.Request.Method))
         {
             if (string.IsNullOrEmpty(_notFoundUrl))
             {
diff --git a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.Streaming.cs b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.Streaming.cs
index e78a938fc9d6..e5bdfe960757 100644
--- a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.Streaming.cs
+++ b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.Streaming.cs
@@ -240,7 +240,7 @@ private static void HandleNavigationAfterResponseStarted(TextWriter writer, Http
 
     private static void WriteResponseTemplate(TextWriter writer, HttpContext httpContext, string destinationUrl)
     {
-        if (string.Equals(httpContext.Request.Method, "POST", StringComparison.OrdinalIgnoreCase))
+        if (HttpMethods.IsPost(httpContext.Request.Method))
         {
             writer.Write(" from=\"form-post\"");
         }
diff --git a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs
index 1f26984a206b..f5d0699e1efe 100644
--- a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs
+++ b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.cs
@@ -85,11 +85,10 @@ internal async Task InitializeStandardComponentServicesAsync(
         var navigationManager = httpContext.RequestServices.GetRequiredService<NavigationManager>();
         ((IHostEnvironmentNavigationManager)navigationManager)?.Initialize(GetContextBaseUri(httpContext.Request), GetFullUri(httpContext.Request), OnNavigateTo);
 
-        if (navigationManager != null)
+        navigationManager?.OnNotFound += (sender, args) =>
         {
-            navigationManager.OnNotFound += async (sender, args) =>
-                await GetErrorHandledTask(SetNotFoundResponseAsync(navigationManager.BaseUri, args));
-        }
+            _ = GetErrorHandledTask(SetNotFoundResponseAsync(navigationManager.BaseUri, args));
+        };
 
         var authenticationStateProvider = httpContext.RequestServices.GetService<AuthenticationStateProvider>();
         if (authenticationStateProvider is IHostEnvironmentAuthenticationStateProvider hostEnvironmentAuthenticationStateProvider)

From 29b5322ce7fa5517c617b62956234d2c6c361f00 Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Mon, 16 Jun 2025 14:00:19 +0200
Subject: [PATCH 25/28] Use existing `TestContentPackage` as a shared project.

---
 .../Components.Shared/Components.Shared.csproj  | 17 -----------------
 .../testassets/Components.Shared/_Imports.razor |  1 -
 .../Components.TestServer.csproj                |  1 -
 ...rComponentEndpointsNoInteractivityStartup.cs |  4 ++--
 .../RazorComponentEndpointsStartup.cs           |  2 +-
 .../RazorComponents/App.razor                   |  4 ++--
 .../Components.WasmMinimal.csproj               |  1 -
 .../PageThatSetsNotFound-Interactive.razor      |  2 +-
 .../Components.WasmMinimal/Routes.razor         |  4 ++--
 .../NotFound}/ComponentThatPostsNotFound.razor  |  0
 .../NotFound}/ComponentThatSetsNotFound.razor   |  0
 .../NotFound}/CustomNotFoundPage.razor          |  0
 .../NotFound}/Index.razor                       |  0
 .../NotFound}/NotFoundLayout.razor              |  0
 .../NotFound}/NotFoundPage.razor                |  0
 .../PageThatPostsNotFound-no-streaming.razor    |  2 +-
 .../PageThatPostsNotFound-streaming.razor       |  2 +-
 .../PageThatSetsNotFound-no-streaming.razor     |  2 +-
 .../PageThatSetsNotFound-streaming.razor        |  2 +-
 .../NotFound}/ReexecutedPage.razor              |  0
 20 files changed, 12 insertions(+), 32 deletions(-)
 delete mode 100644 src/Components/test/testassets/Components.Shared/Components.Shared.csproj
 delete mode 100644 src/Components/test/testassets/Components.Shared/_Imports.razor
 rename src/Components/test/testassets/{Components.Shared => TestContentPackage/NotFound}/ComponentThatPostsNotFound.razor (100%)
 rename src/Components/test/testassets/{Components.Shared => TestContentPackage/NotFound}/ComponentThatSetsNotFound.razor (100%)
 rename src/Components/test/testassets/{Components.Shared => TestContentPackage/NotFound}/CustomNotFoundPage.razor (100%)
 rename src/Components/test/testassets/{Components.Shared => TestContentPackage/NotFound}/Index.razor (100%)
 rename src/Components/test/testassets/{Components.Shared => TestContentPackage/NotFound}/NotFoundLayout.razor (100%)
 rename src/Components/test/testassets/{Components.Shared => TestContentPackage/NotFound}/NotFoundPage.razor (100%)
 rename src/Components/test/testassets/{Components.Shared => TestContentPackage/NotFound}/PageThatPostsNotFound-no-streaming.razor (84%)
 rename src/Components/test/testassets/{Components.Shared => TestContentPackage/NotFound}/PageThatPostsNotFound-streaming.razor (81%)
 rename src/Components/test/testassets/{Components.Shared => TestContentPackage/NotFound}/PageThatSetsNotFound-no-streaming.razor (85%)
 rename src/Components/test/testassets/{Components.Shared => TestContentPackage/NotFound}/PageThatSetsNotFound-streaming.razor (81%)
 rename src/Components/test/testassets/{Components.Shared => TestContentPackage/NotFound}/ReexecutedPage.razor (100%)

diff --git a/src/Components/test/testassets/Components.Shared/Components.Shared.csproj b/src/Components/test/testassets/Components.Shared/Components.Shared.csproj
deleted file mode 100644
index e2b71dba2459..000000000000
--- a/src/Components/test/testassets/Components.Shared/Components.Shared.csproj
+++ /dev/null
@@ -1,17 +0,0 @@
-<Project Sdk="Microsoft.NET.Sdk.Razor">
-
-  <PropertyGroup>
-    <TargetFramework>$(DefaultNetCoreTargetFramework)</TargetFramework>
-    <Nullable>enable</Nullable>
-    <ImplicitUsings>enable</ImplicitUsings>
-  </PropertyGroup>
-
-  <ItemGroup>
-    <SupportedPlatform Include="browser" />
-  </ItemGroup>
-
-  <ItemGroup>
-    <Reference Include="Microsoft.AspNetCore.Components.Web" />
-  </ItemGroup>
-
-</Project>
\ No newline at end of file
diff --git a/src/Components/test/testassets/Components.Shared/_Imports.razor b/src/Components/test/testassets/Components.Shared/_Imports.razor
deleted file mode 100644
index 2b9557ce5a53..000000000000
--- a/src/Components/test/testassets/Components.Shared/_Imports.razor
+++ /dev/null
@@ -1 +0,0 @@
-@using Microsoft.AspNetCore.Components.Web
\ No newline at end of file
diff --git a/src/Components/test/testassets/Components.TestServer/Components.TestServer.csproj b/src/Components/test/testassets/Components.TestServer/Components.TestServer.csproj
index 7f14796efe56..b7d6d2b019b8 100644
--- a/src/Components/test/testassets/Components.TestServer/Components.TestServer.csproj
+++ b/src/Components/test/testassets/Components.TestServer/Components.TestServer.csproj
@@ -36,7 +36,6 @@
     <ProjectReference Include="..\BasicTestApp\BasicTestApp.csproj" />
     <ProjectReference Include="..\Components.WasmMinimal\Components.WasmMinimal.csproj" />
     <ProjectReference Include="..\Components.WasmRemoteAuthentication\Components.WasmRemoteAuthentication.csproj" />
-    <ProjectReference Include="..\Components.Shared\Components.Shared.csproj" />
   </ItemGroup>
 
   <ItemGroup>
diff --git a/src/Components/test/testassets/Components.TestServer/RazorComponentEndpointsNoInteractivityStartup.cs b/src/Components/test/testassets/Components.TestServer/RazorComponentEndpointsNoInteractivityStartup.cs
index 6bc67a18c6fc..92fe46b53a3a 100644
--- a/src/Components/test/testassets/Components.TestServer/RazorComponentEndpointsNoInteractivityStartup.cs
+++ b/src/Components/test/testassets/Components.TestServer/RazorComponentEndpointsNoInteractivityStartup.cs
@@ -72,7 +72,7 @@ public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
                 reexecutionApp.UseEndpoints(endpoints =>
                 {
                     endpoints.MapRazorComponents<TRootComponent>()
-                        .AddAdditionalAssemblies(Assembly.Load("Components.Shared"));
+                        .AddAdditionalAssemblies(Assembly.Load("TestContentPackage"));
                 });
             });
 
@@ -94,7 +94,7 @@ private void ConfigureSubdirPipeline(IApplicationBuilder app, IWebHostEnvironmen
         app.UseEndpoints(endpoints =>
         {
             endpoints.MapRazorComponents<TRootComponent>()
-                .AddAdditionalAssemblies(Assembly.Load("Components.Shared"));
+                .AddAdditionalAssemblies(Assembly.Load("TestContentPackage"));
         });
     }
 }
diff --git a/src/Components/test/testassets/Components.TestServer/RazorComponentEndpointsStartup.cs b/src/Components/test/testassets/Components.TestServer/RazorComponentEndpointsStartup.cs
index 2eccc4f661fb..dacdc2b72b5a 100644
--- a/src/Components/test/testassets/Components.TestServer/RazorComponentEndpointsStartup.cs
+++ b/src/Components/test/testassets/Components.TestServer/RazorComponentEndpointsStartup.cs
@@ -146,7 +146,7 @@ private void ConfigureEndpoints(IApplicationBuilder app, IWebHostEnvironment env
             }
 
             _ = endpoints.MapRazorComponents<TRootComponent>()
-                .AddAdditionalAssemblies(Assembly.Load("Components.Shared"))
+                .AddAdditionalAssemblies(Assembly.Load("TestContentPackage"))
                 .AddAdditionalAssemblies(Assembly.Load("Components.WasmMinimal"))
                 .AddInteractiveServerRenderMode(options =>
                 {
diff --git a/src/Components/test/testassets/Components.TestServer/RazorComponents/App.razor b/src/Components/test/testassets/Components.TestServer/RazorComponents/App.razor
index dc696f1a0f2d..2219da3955d2 100644
--- a/src/Components/test/testassets/Components.TestServer/RazorComponents/App.razor
+++ b/src/Components/test/testassets/Components.TestServer/RazorComponents/App.razor
@@ -1,6 +1,6 @@
 @using Components.TestServer.RazorComponents.Pages.Forms
 @using Components.WasmMinimal.Pages.NotFound
-@using Components.Shared
+@using TestContentPackage.NotFound
 
 @code {
     [Parameter]
@@ -30,7 +30,7 @@
     <HeadOutlet />
 </head>
 <body>
-    <Router AppAssembly="@typeof(App).Assembly" AdditionalAssemblies="new[] { typeof(Components.Shared.NotFoundPage).Assembly }" NotFoundPage="NotFoundPageType">
+    <Router AppAssembly="@typeof(App).Assembly" AdditionalAssemblies="new[] { typeof(TestContentPackage.NotFound.NotFoundPage).Assembly }" NotFoundPage="NotFoundPageType">
         <Found Context="routeData">
             <RouteView RouteData="@routeData" />
             <FocusOnNavigate RouteData="@routeData" Selector="[data-focus-on-navigate]" />
diff --git a/src/Components/test/testassets/Components.WasmMinimal/Components.WasmMinimal.csproj b/src/Components/test/testassets/Components.WasmMinimal/Components.WasmMinimal.csproj
index ecacf8c5078d..deabcd7fccae 100644
--- a/src/Components/test/testassets/Components.WasmMinimal/Components.WasmMinimal.csproj
+++ b/src/Components/test/testassets/Components.WasmMinimal/Components.WasmMinimal.csproj
@@ -16,7 +16,6 @@
   <ItemGroup>
     <ProjectReference Include="..\NotReferencedInWasmCodePackage\NotReferencedInWasmCodePackage.csproj" />
     <ProjectReference Include="..\TestContentPackage\TestContentPackage.csproj" />
-    <ProjectReference Include="..\Components.Shared\Components.Shared.csproj" />
   </ItemGroup>
 
 </Project>
diff --git a/src/Components/test/testassets/Components.WasmMinimal/Pages/NotFound/PageThatSetsNotFound-Interactive.razor b/src/Components/test/testassets/Components.WasmMinimal/Pages/NotFound/PageThatSetsNotFound-Interactive.razor
index 46c0c65cecf9..fe0362ccda63 100644
--- a/src/Components/test/testassets/Components.WasmMinimal/Pages/NotFound/PageThatSetsNotFound-Interactive.razor
+++ b/src/Components/test/testassets/Components.WasmMinimal/Pages/NotFound/PageThatSetsNotFound-Interactive.razor
@@ -6,7 +6,7 @@
     the component's content will be rendered when it becomes interactive
 *@
 
-<Components.Shared.ComponentThatSetsNotFound @rendermode="@RenderModeHelper.GetRenderMode(_renderMode)" WaitForInteractivity="true" />
+<TestContentPackage.NotFound.ComponentThatSetsNotFound @rendermode="@RenderModeHelper.GetRenderMode(_renderMode)" WaitForInteractivity="true" />
 
 @code{
     [Parameter, SupplyParameterFromQuery(Name = "renderMode")]
diff --git a/src/Components/test/testassets/Components.WasmMinimal/Routes.razor b/src/Components/test/testassets/Components.WasmMinimal/Routes.razor
index 5a2ba90aaed4..010176d437e7 100644
--- a/src/Components/test/testassets/Components.WasmMinimal/Routes.razor
+++ b/src/Components/test/testassets/Components.WasmMinimal/Routes.razor
@@ -1,6 +1,6 @@
 @using Microsoft.AspNetCore.Components.Routing
 @using Components.WasmMinimal.Pages.NotFound
-@using Components.Shared
+@using TestContentPackage.NotFound
 @inject NavigationManager NavigationManager
 
 @code {
@@ -23,7 +23,7 @@
     }
 }
 
-<Router AppAssembly="@typeof(Program).Assembly" AdditionalAssemblies="new[] { typeof(Components.Shared.NotFoundPage).Assembly }" NotFoundPage="NotFoundPageType">
+<Router AppAssembly="@typeof(Program).Assembly" AdditionalAssemblies="new[] { typeof(TestContentPackage.NotFound.NotFoundPage).Assembly }" NotFoundPage="NotFoundPageType">
     <Found Context="routeData">
         <RouteView RouteData="@routeData" />
         <FocusOnNavigate RouteData="@routeData" Selector="h1" />
diff --git a/src/Components/test/testassets/Components.Shared/ComponentThatPostsNotFound.razor b/src/Components/test/testassets/TestContentPackage/NotFound/ComponentThatPostsNotFound.razor
similarity index 100%
rename from src/Components/test/testassets/Components.Shared/ComponentThatPostsNotFound.razor
rename to src/Components/test/testassets/TestContentPackage/NotFound/ComponentThatPostsNotFound.razor
diff --git a/src/Components/test/testassets/Components.Shared/ComponentThatSetsNotFound.razor b/src/Components/test/testassets/TestContentPackage/NotFound/ComponentThatSetsNotFound.razor
similarity index 100%
rename from src/Components/test/testassets/Components.Shared/ComponentThatSetsNotFound.razor
rename to src/Components/test/testassets/TestContentPackage/NotFound/ComponentThatSetsNotFound.razor
diff --git a/src/Components/test/testassets/Components.Shared/CustomNotFoundPage.razor b/src/Components/test/testassets/TestContentPackage/NotFound/CustomNotFoundPage.razor
similarity index 100%
rename from src/Components/test/testassets/Components.Shared/CustomNotFoundPage.razor
rename to src/Components/test/testassets/TestContentPackage/NotFound/CustomNotFoundPage.razor
diff --git a/src/Components/test/testassets/Components.Shared/Index.razor b/src/Components/test/testassets/TestContentPackage/NotFound/Index.razor
similarity index 100%
rename from src/Components/test/testassets/Components.Shared/Index.razor
rename to src/Components/test/testassets/TestContentPackage/NotFound/Index.razor
diff --git a/src/Components/test/testassets/Components.Shared/NotFoundLayout.razor b/src/Components/test/testassets/TestContentPackage/NotFound/NotFoundLayout.razor
similarity index 100%
rename from src/Components/test/testassets/Components.Shared/NotFoundLayout.razor
rename to src/Components/test/testassets/TestContentPackage/NotFound/NotFoundLayout.razor
diff --git a/src/Components/test/testassets/Components.Shared/NotFoundPage.razor b/src/Components/test/testassets/TestContentPackage/NotFound/NotFoundPage.razor
similarity index 100%
rename from src/Components/test/testassets/Components.Shared/NotFoundPage.razor
rename to src/Components/test/testassets/TestContentPackage/NotFound/NotFoundPage.razor
diff --git a/src/Components/test/testassets/Components.Shared/PageThatPostsNotFound-no-streaming.razor b/src/Components/test/testassets/TestContentPackage/NotFound/PageThatPostsNotFound-no-streaming.razor
similarity index 84%
rename from src/Components/test/testassets/Components.Shared/PageThatPostsNotFound-no-streaming.razor
rename to src/Components/test/testassets/TestContentPackage/NotFound/PageThatPostsNotFound-no-streaming.razor
index d02932a8c4e5..4aa3a245b047 100644
--- a/src/Components/test/testassets/Components.Shared/PageThatPostsNotFound-no-streaming.razor
+++ b/src/Components/test/testassets/TestContentPackage/NotFound/PageThatPostsNotFound-no-streaming.razor
@@ -8,4 +8,4 @@
     interactive later if interactivity was enabled in the app
 *@
 
-<Components.Shared.ComponentThatPostsNotFound />
\ No newline at end of file
+<TestContentPackage.NotFound.ComponentThatPostsNotFound />
\ No newline at end of file
diff --git a/src/Components/test/testassets/Components.Shared/PageThatPostsNotFound-streaming.razor b/src/Components/test/testassets/TestContentPackage/NotFound/PageThatPostsNotFound-streaming.razor
similarity index 81%
rename from src/Components/test/testassets/Components.Shared/PageThatPostsNotFound-streaming.razor
rename to src/Components/test/testassets/TestContentPackage/NotFound/PageThatPostsNotFound-streaming.razor
index 93ac9fc23359..a542c35c52f4 100644
--- a/src/Components/test/testassets/Components.Shared/PageThatPostsNotFound-streaming.razor
+++ b/src/Components/test/testassets/TestContentPackage/NotFound/PageThatPostsNotFound-streaming.razor
@@ -8,4 +8,4 @@
     interactive later if interactivity was enabled in the app
 *@
 
-<Components.Shared.ComponentThatPostsNotFound StartStreaming="true"/>
\ No newline at end of file
+<TestContentPackage.NotFound.ComponentThatPostsNotFound StartStreaming="true"/>
\ No newline at end of file
diff --git a/src/Components/test/testassets/Components.Shared/PageThatSetsNotFound-no-streaming.razor b/src/Components/test/testassets/TestContentPackage/NotFound/PageThatSetsNotFound-no-streaming.razor
similarity index 85%
rename from src/Components/test/testassets/Components.Shared/PageThatSetsNotFound-no-streaming.razor
rename to src/Components/test/testassets/TestContentPackage/NotFound/PageThatSetsNotFound-no-streaming.razor
index 6ac4db557e13..b99ed19711d7 100644
--- a/src/Components/test/testassets/Components.Shared/PageThatSetsNotFound-no-streaming.razor
+++ b/src/Components/test/testassets/TestContentPackage/NotFound/PageThatSetsNotFound-no-streaming.razor
@@ -8,4 +8,4 @@
     interactive later if interactivity was enabled in the app
 *@
 
-<Components.Shared.ComponentThatSetsNotFound />
\ No newline at end of file
+<TestContentPackage.NotFound.ComponentThatSetsNotFound />
\ No newline at end of file
diff --git a/src/Components/test/testassets/Components.Shared/PageThatSetsNotFound-streaming.razor b/src/Components/test/testassets/TestContentPackage/NotFound/PageThatSetsNotFound-streaming.razor
similarity index 81%
rename from src/Components/test/testassets/Components.Shared/PageThatSetsNotFound-streaming.razor
rename to src/Components/test/testassets/TestContentPackage/NotFound/PageThatSetsNotFound-streaming.razor
index 061d35386bc0..e3124758ce65 100644
--- a/src/Components/test/testassets/Components.Shared/PageThatSetsNotFound-streaming.razor
+++ b/src/Components/test/testassets/TestContentPackage/NotFound/PageThatSetsNotFound-streaming.razor
@@ -8,4 +8,4 @@
     interactive later if interactivity was enabled in the app
 *@
 
-<Components.Shared.ComponentThatSetsNotFound StartStreaming="true"/>
\ No newline at end of file
+<TestContentPackage.NotFound.ComponentThatSetsNotFound StartStreaming="true"/>
\ No newline at end of file
diff --git a/src/Components/test/testassets/Components.Shared/ReexecutedPage.razor b/src/Components/test/testassets/TestContentPackage/NotFound/ReexecutedPage.razor
similarity index 100%
rename from src/Components/test/testassets/Components.Shared/ReexecutedPage.razor
rename to src/Components/test/testassets/TestContentPackage/NotFound/ReexecutedPage.razor

From c0cee2f2f34204bf10dbfb0e803b41957268d15b Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Mon, 16 Jun 2025 14:52:34 +0200
Subject: [PATCH 26/28] Fix client navigation: always use enhanced nav for not
 found rendering, unless user explicitly disbled it.

---
 .../EndpointHtmlRenderer.Streaming.cs         |  9 ++--
 .../src/Rendering/StreamingRendering.ts       | 11 +++--
 .../NoInteractivityTest.cs                    | 44 ++-----------------
 src/Shared/E2ETesting/WaitAssert.cs           |  3 --
 4 files changed, 15 insertions(+), 52 deletions(-)

diff --git a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.Streaming.cs b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.Streaming.cs
index e5bdfe960757..926aa42fe8d8 100644
--- a/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.Streaming.cs
+++ b/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.Streaming.cs
@@ -229,23 +229,24 @@ private static void HandleExceptionAfterResponseStarted(HttpContext httpContext,
     private static void HandleNotFoundAfterResponseStarted(TextWriter writer, HttpContext httpContext, string notFoundUrl)
     {
         writer.Write("<blazor-ssr><template type=\"not-found\"");
-        WriteResponseTemplate(writer, httpContext, notFoundUrl);
+        WriteResponseTemplate(writer, httpContext, notFoundUrl, useEnhancedNav: true);
     }
 
     private static void HandleNavigationAfterResponseStarted(TextWriter writer, HttpContext httpContext, string destinationUrl)
     {
         writer.Write("<blazor-ssr><template type=\"redirection\"");
-        WriteResponseTemplate(writer, httpContext, destinationUrl);
+        bool useEnhancedNav = IsProgressivelyEnhancedNavigation(httpContext.Request);
+        WriteResponseTemplate(writer, httpContext, destinationUrl, useEnhancedNav);
     }
 
-    private static void WriteResponseTemplate(TextWriter writer, HttpContext httpContext, string destinationUrl)
+    private static void WriteResponseTemplate(TextWriter writer, HttpContext httpContext, string destinationUrl, bool useEnhancedNav)
     {
         if (HttpMethods.IsPost(httpContext.Request.Method))
         {
             writer.Write(" from=\"form-post\"");
         }
 
-        if (IsProgressivelyEnhancedNavigation(httpContext.Request))
+        if (useEnhancedNav)
         {
             writer.Write(" enhanced=\"true\"");
         }
diff --git a/src/Components/Web.JS/src/Rendering/StreamingRendering.ts b/src/Components/Web.JS/src/Rendering/StreamingRendering.ts
index 46a5a6dd9ed4..7ac02952068e 100644
--- a/src/Components/Web.JS/src/Rendering/StreamingRendering.ts
+++ b/src/Components/Web.JS/src/Rendering/StreamingRendering.ts
@@ -45,12 +45,16 @@ class BlazorStreamingUpdate extends HTMLElement {
             insertStreamingContentIntoDocument(componentId, node.content);
           }
         } else {
+          const isEnhancedNav = node.getAttribute('enhanced') === 'true';
           switch (node.getAttribute('type')) {
             case 'redirection':
-              redirect(node, true);
+              redirect(node, true, isEnhancedNav);
               break;
             case 'not-found':
-              redirect(node, false);
+              // not-found template has enhanced nav set to true by default,
+              // check for the options to avoid overriding user settings
+              const useEnhancedNav = isEnhancedNav && enableDomPreservation;
+              redirect(node, false, useEnhancedNav);
               break;
             case 'error':
               // This is kind of brutal but matches what happens without progressive enhancement
@@ -63,12 +67,11 @@ class BlazorStreamingUpdate extends HTMLElement {
   }
 }
 
-function redirect(node: HTMLTemplateElement, changeUrl: boolean): void {
+function redirect(node: HTMLTemplateElement, changeUrl: boolean, isEnhancedNav: boolean): void {
   // We use 'replace' here because it's closest to the non-progressively-enhanced behavior, and will make the most sense
  // if the async delay was very short, as the user would not perceive having been on the intermediate page.
  const destinationUrl = toAbsoluteUri(node.content.textContent!);
  const isFormPost = node.getAttribute('from') === 'form-post';
- const isEnhancedNav = node.getAttribute('enhanced') === 'true';
  if (isEnhancedNav && isWithinBaseUriSpace(destinationUrl)) {
    // At this point the destinationUrl might be an opaque URL so we don't know whether it's internal/external or
    // whether it's even going to the same URL we're currently on. So we don't know how to update the history.
diff --git a/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs b/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
index 760db83fb3eb..d9afcc9e35f1 100644
--- a/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
+++ b/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
@@ -127,16 +127,11 @@ private void AssertNotFoundPageRendered()
     private void AssertUrlNotChanged(string expectedUrl) =>
         Browser.True(() => Browser.Url.Contains(expectedUrl), $"Expected URL to contain '{expectedUrl}', but found '{Browser.Url}'");
 
-    private void AssertUrlChanged(string urlPart) =>
-        Browser.False(() => Browser.Url.Contains(urlPart), $"Expected URL not to contain '{urlPart}', but found '{Browser.Url}'");
-
     [Theory]
     [InlineData(true, true)]
     [InlineData(true, false)]
     [InlineData(false, true)]
     [InlineData(false, false)]
-    // When response has not started, it does not matter how we arrive the the page setting the not found status code.
-    // We can navigate straight to the testing page, skipping the index page.
     public void NotFoundSetOnInitialization_ResponseNotStarted_SSR(bool hasReExecutionMiddleware, bool hasCustomNotFoundPageSet)
     {
         string reexecution = hasReExecutionMiddleware ? "/reexecution" : "";
@@ -154,32 +149,18 @@ public void NotFoundSetOnInitialization_ResponseNotStarted_SSR(bool hasReExecuti
         AssertUrlNotChanged(testUrl);
     }
 
-    // If the response has started, it matters how we arrive the the page setting the not found status code
-    // because we are rendering client-side. In browser-initiated navigation and form submissions, the headers
-    // don't contain enhanced-nav and we redirect to the not found page. In link clicking navigation,
-    // the headers contain enhanced-nav, redirection is not needed and the original url is preserved.
-
     [Theory]
     [InlineData(true, true)]
     [InlineData(true, false)]
     [InlineData(false, true)]
     [InlineData(false, false)]
-    // enhanced navigation is switched off for browser navigation
     public void NotFoundSetOnInitialization_ResponseStarted_BrowserNavigation_SSR(bool hasReExecutionMiddleware, bool hasCustomNotFoundPageSet)
     {
         string reexecution = hasReExecutionMiddleware ? "/reexecution" : "";
         string testUrl = $"{ServerPathBase}{reexecution}/set-not-found-ssr-streaming?useCustomNotFoundPage={hasCustomNotFoundPageSet}";
         Navigate(testUrl);
         AssertNotFoundRendered_ResponseStarted_Or_POST(hasReExecutionMiddleware, hasCustomNotFoundPageSet, testUrl);
-        bool throwsException = !hasCustomNotFoundPageSet && !hasReExecutionMiddleware;
-        if (throwsException)
-        {
-            AssertUrlNotChanged(testUrl);
-        }
-        else
-        {
-            AssertUrlChanged(testUrl);
-        }
+        AssertUrlNotChanged(testUrl);
     }
 
     private void AssertNotFoundRendered_ResponseStarted_Or_POST(bool hasReExecutionMiddleware, bool hasCustomNotFoundPageSet, string testUrl)
@@ -204,7 +185,6 @@ private void AssertNotFoundRendered_ResponseStarted_Or_POST(bool hasReExecutionM
     [InlineData(true, false)]
     [InlineData(false, true)]
     [InlineData(false, false)]
-    // enhanced navigation is switched on for link navigation
     public void NotFoundSetOnInitialization_ResponseStarted_LinkNavigation_SSR(bool hasReExecutionMiddleware, bool hasCustomNotFoundPageSet)
     {
         string testUrl = NavigateByLinkToPageTestingNotFound("Sets", hasReExecutionMiddleware, hasCustomNotFoundPageSet);
@@ -229,8 +209,6 @@ private string NavigateByLinkToPageTestingNotFound(string action, bool hasReExec
     [InlineData(true, false)]
     [InlineData(false, true)]
     [InlineData(false, false)]
-    // NotFound triggered by POST cannot get rendered in the same batch and we rely on the client to render it
-    // However, because it is triggered by form POST, it won't have enhanced-nav headers
     public void NotFoundSetOnFormSubmit_ResponseNotStarted_SSR(bool hasReExecutionMiddleware, bool hasCustomNotFoundPageSet)
     {
         string reexecution = hasReExecutionMiddleware ? "/reexecution" : "";
@@ -239,15 +217,7 @@ public void NotFoundSetOnFormSubmit_ResponseNotStarted_SSR(bool hasReExecutionMi
         Browser.FindElement(By.Id("not-found-form")).FindElement(By.TagName("button")).Click();
 
         AssertNotFoundRendered_ResponseStarted_Or_POST(hasReExecutionMiddleware, hasCustomNotFoundPageSet, testUrl);
-        bool throwsException = !hasCustomNotFoundPageSet && !hasReExecutionMiddleware;
-        if (throwsException)
-        {
-            AssertUrlNotChanged(testUrl);
-        }
-        else
-        {
-            AssertUrlChanged(testUrl);
-        }
+        AssertUrlNotChanged(testUrl);
     }
 
     [Theory]
@@ -263,15 +233,7 @@ public void NotFoundSetOnFormSubmit_ResponseStarted_SSR(bool hasReExecutionMiddl
         Browser.FindElement(By.Id("not-found-form")).FindElement(By.TagName("button")).Click();
 
         AssertNotFoundRendered_ResponseStarted_Or_POST(hasReExecutionMiddleware, hasCustomNotFoundPageSet, testUrl);
-        bool throwsException = !hasCustomNotFoundPageSet && !hasReExecutionMiddleware;
-        if (throwsException)
-        {
-            AssertUrlNotChanged(testUrl);
-        }
-        else
-        {
-            AssertUrlChanged(testUrl);
-        }
+        AssertUrlNotChanged(testUrl);
     }
 
     private void AssertNotFoundFragmentRendered() =>
diff --git a/src/Shared/E2ETesting/WaitAssert.cs b/src/Shared/E2ETesting/WaitAssert.cs
index a3bb53520f30..f10953c95e3d 100644
--- a/src/Shared/E2ETesting/WaitAssert.cs
+++ b/src/Shared/E2ETesting/WaitAssert.cs
@@ -40,9 +40,6 @@ public static void True(this IWebDriver driver, Func<bool> actual, TimeSpan time
     public static void True(this IWebDriver driver, Func<bool> actual, string message)
         => WaitAssertCore(driver, () => Assert.True(actual(), message));
 
-    public static void False(this IWebDriver driver, Func<bool> actual, string message)
-        => WaitAssertCore(driver, () => Assert.False(actual(), message));
-
     public static void False(this IWebDriver driver, Func<bool> actual)
         => WaitAssertCore(driver, () => Assert.False(actual()));
 

From 975e2507fe75d236fe532e69f449f707d62ea75f Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Mon, 16 Jun 2025 14:58:46 +0200
Subject: [PATCH 27/28] Remove test duplication: when server always requests a
 render with enhanced nav, actual request headers do not matter.

---
 .../NoInteractivityTest.cs                    | 26 +------------------
 1 file changed, 1 insertion(+), 25 deletions(-)

diff --git a/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs b/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
index d9afcc9e35f1..23f5cf0c9d73 100644
--- a/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
+++ b/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
@@ -154,7 +154,7 @@ public void NotFoundSetOnInitialization_ResponseNotStarted_SSR(bool hasReExecuti
     [InlineData(true, false)]
     [InlineData(false, true)]
     [InlineData(false, false)]
-    public void NotFoundSetOnInitialization_ResponseStarted_BrowserNavigation_SSR(bool hasReExecutionMiddleware, bool hasCustomNotFoundPageSet)
+    public void NotFoundSetOnInitialization_ResponseStarted_SSR(bool hasReExecutionMiddleware, bool hasCustomNotFoundPageSet)
     {
         string reexecution = hasReExecutionMiddleware ? "/reexecution" : "";
         string testUrl = $"{ServerPathBase}{reexecution}/set-not-found-ssr-streaming?useCustomNotFoundPage={hasCustomNotFoundPageSet}";
@@ -180,30 +180,6 @@ private void AssertNotFoundRendered_ResponseStarted_Or_POST(bool hasReExecutionM
         }
     }
 
-    [Theory]
-    [InlineData(true, true)]
-    [InlineData(true, false)]
-    [InlineData(false, true)]
-    [InlineData(false, false)]
-    public void NotFoundSetOnInitialization_ResponseStarted_LinkNavigation_SSR(bool hasReExecutionMiddleware, bool hasCustomNotFoundPageSet)
-    {
-        string testUrl = NavigateByLinkToPageTestingNotFound("Sets", hasReExecutionMiddleware, hasCustomNotFoundPageSet);
-        AssertNotFoundRendered_ResponseStarted_Or_POST(hasReExecutionMiddleware, hasCustomNotFoundPageSet, testUrl);
-        AssertUrlNotChanged(testUrl);
-    }
-
-    private string NavigateByLinkToPageTestingNotFound(string action, bool hasReExecutionMiddleware, bool hasCustomNotFoundPageSet)
-    {
-        string reexecution = hasReExecutionMiddleware ? "/reexecution" : "";
-        Navigate($"{ServerPathBase}/not-found-index?useCustomNotFoundPage={hasCustomNotFoundPageSet}");
-        Browser.Equal("List of Not Found test pages", () => Browser.FindElement(By.Id("test-info")).Text);
-        string reexecutionText = hasReExecutionMiddleware ? " with reexecution" : "";
-        var link = Browser.FindElement(By.LinkText($"PageThat{action}NotFound-streaming{reexecutionText}"));
-        var testUrl = link.GetAttribute("href");
-        link.Click();
-        return testUrl;
-    }
-
     [Theory]
     [InlineData(true, true)]
     [InlineData(true, false)]

From 9a926ce175ad79e65101c2f1a9d7b61851f2433b Mon Sep 17 00:00:00 2001
From: Ilona Tomkowicz <itomkowicz@microsoft.com>
Date: Mon, 16 Jun 2025 15:06:11 +0200
Subject: [PATCH 28/28] Use client redirect with url change when user dissbles
 enhanced navigation.

---
 .../ServerRenderingTests/NoInteractivityTest.cs | 17 +++++++++++++++++
 src/Shared/E2ETesting/WaitAssert.cs             |  3 +++
 2 files changed, 20 insertions(+)

diff --git a/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs b/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
index 23f5cf0c9d73..e5a438283f84 100644
--- a/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
+++ b/src/Components/test/E2ETest/ServerRenderingTests/NoInteractivityTest.cs
@@ -127,6 +127,9 @@ private void AssertNotFoundPageRendered()
     private void AssertUrlNotChanged(string expectedUrl) =>
         Browser.True(() => Browser.Url.Contains(expectedUrl), $"Expected URL to contain '{expectedUrl}', but found '{Browser.Url}'");
 
+    private void AssertUrlChanged(string urlPart) =>
+        Browser.False(() => Browser.Url.Contains(urlPart), $"Expected URL not to contain '{urlPart}', but found '{Browser.Url}'");
+
     [Theory]
     [InlineData(true, true)]
     [InlineData(true, false)]
@@ -163,6 +166,20 @@ public void NotFoundSetOnInitialization_ResponseStarted_SSR(bool hasReExecutionM
         AssertUrlNotChanged(testUrl);
     }
 
+    [Theory]
+    [InlineData(true, true)]
+    [InlineData(true, false)]
+    [InlineData(false, true)]
+    public void NotFoundSetOnInitialization_ResponseStarted_EnhancedNavigationDisabled_SSR(bool hasReExecutionMiddleware, bool hasCustomNotFoundPageSet)
+    {
+        EnhancedNavigationTestUtil.SuppressEnhancedNavigation(this, true, skipNavigation: true);
+        string reexecution = hasReExecutionMiddleware ? "/reexecution" : "";
+        string testUrl = $"{ServerPathBase}{reexecution}/set-not-found-ssr-streaming?useCustomNotFoundPage={hasCustomNotFoundPageSet}";
+        Navigate(testUrl);
+        AssertNotFoundRendered_ResponseStarted_Or_POST(hasReExecutionMiddleware, hasCustomNotFoundPageSet, testUrl);
+        AssertUrlChanged(testUrl);
+    }
+
     private void AssertNotFoundRendered_ResponseStarted_Or_POST(bool hasReExecutionMiddleware, bool hasCustomNotFoundPageSet, string testUrl)
     {
         if (hasCustomNotFoundPageSet)
diff --git a/src/Shared/E2ETesting/WaitAssert.cs b/src/Shared/E2ETesting/WaitAssert.cs
index f10953c95e3d..a3bb53520f30 100644
--- a/src/Shared/E2ETesting/WaitAssert.cs
+++ b/src/Shared/E2ETesting/WaitAssert.cs
@@ -40,6 +40,9 @@ public static void True(this IWebDriver driver, Func<bool> actual, TimeSpan time
     public static void True(this IWebDriver driver, Func<bool> actual, string message)
         => WaitAssertCore(driver, () => Assert.True(actual(), message));
 
+    public static void False(this IWebDriver driver, Func<bool> actual, string message)
+        => WaitAssertCore(driver, () => Assert.False(actual(), message));
+
     public static void False(this IWebDriver driver, Func<bool> actual)
         => WaitAssertCore(driver, () => Assert.False(actual()));