Ensure enhanced nav requests have the correct headers (#50263)

Author: SteveSandersonMS

src/Components/Web.JS/dist/Release/blazor.web.js

@@ -133,7 +133,10 @@ export async function performEnhancedPageLoad(internalDestinationHref: string, f
   const abortSignal = currentEnhancedNavigationAbortController.signal;
   const responsePromise = fetch(internalDestinationHref, Object.assign({
     signal: abortSignal,
-    headers: { 'blazor-enhanced-nav': 'on' },
+    headers: {
+      'blazor-enhanced-nav': 'on',
+      'accept': 'text/html',
+    },
   }, fetchOptions));
   await getResponsePartsWithFraming(responsePromise, abortSignal,
     (response, initialContent) => {

src/Components/Web.JS/src/Services/NavigationEnhancement.ts

@@ -70,6 +70,24 @@ public void CanNavigateToNonHtmlResponse()
         Browser.Equal("Hello, this is plain text", () => Browser.Exists(By.TagName("html")).Text);
     }
 
+    [Fact]
+    public void EnhancedNavRequestsIncludeExpectedHeaders()
+    {
+        Navigate($"{ServerPathBase}/nav");
+        Browser.Exists(By.TagName("nav")).FindElement(By.LinkText("List headers")).Click();
+
+        var ul = Browser.Exists(By.Id("all-headers"));
+        var allHeaders = ul.FindElements(By.TagName("li")).Select(x => x.Text.ToLowerInvariant()).ToList();
+
+        // The server can trigger arbitrary behavior based on this
+        Assert.Contains("blazor-enhanced-nav: on", allHeaders);
+
+        // This is to make the enhanced nav outcomes more similar to non-enhanced nav.
+        // For example, the default error middleware will only serve the error page if
+        // this header is included.
+        Assert.Contains("accept: text/html", allHeaders);
+    }
+
     [Fact]
     public void ScrollsToHashWithContentAddedAsynchronously()
     {

src/Components/test/E2ETest/ServerRenderingTests/EnhancedNavigationTest.cs

@@ -4,6 +4,7 @@
 using System.Globalization;
 using System.Reflection;
 using System.Security.Claims;
+using System.Web;
 using Components.TestServer.RazorComponents;
 using Components.TestServer.RazorComponents.Pages.Forms;
 using Components.TestServer.Services;
@@ -123,5 +124,17 @@ private static void MapEnhancedNavigationEndpoints(IEndpointRouteBuilder endpoin
             response.ContentType = "text/html";
             await response.WriteAsync("<h1>404</h1><p>Sorry, there's nothing here! This is a custom server-generated 404 message.</p>");
         });
+
+        // Used when testing that enhanced nav includes "Accept: text/html"
+        endpoints.Map("/nav/list-headers", async (HttpRequest request, HttpResponse response) =>
+        {
+            response.ContentType = "text/html";
+            await response.WriteAsync("<ul id='all-headers'>");
+            foreach (var header in request.Headers)
+            {
+                await response.WriteAsync($"<li>{HttpUtility.HtmlEncode(header.Key)}: {HttpUtility.HtmlEncode(header.Value)}</li>");
+            }
+            await response.WriteAsync("</ul>");
+        });
     }
 }

src/Components/test/testassets/Components.TestServer/RazorComponentEndpointsStartup.cs

@@ -12,9 +12,10 @@
     <NavLink href="nav/do-redirection?destination=https://microsoft.com">Redirect external</NavLink> |
     <NavLink href="nav/do-redirection-while-streaming">Redirect while streaming</NavLink> |
     <NavLink href="nav/do-redirection-while-streaming?destination=https://microsoft.com">Redirect external while streaming</NavLink> |
-    <NavLink href="nav/throw-while-streaming">Error while streaming</NavLink>
-    <NavLink href="nav/interactive-component-navigation/server">Interactive component navigation (server)</NavLink>
-    <NavLink href="nav/interactive-component-navigation/webassembly">Interactive component navigation (webassembly)</NavLink>
+    <NavLink href="nav/throw-while-streaming">Error while streaming</NavLink> |
+    <NavLink href="nav/interactive-component-navigation/server">Interactive component navigation (server)</NavLink> |
+    <NavLink href="nav/interactive-component-navigation/webassembly">Interactive component navigation (webassembly)</NavLink> |
+    <NavLink href="nav/list-headers">List headers</NavLink> |
 </nav>
 <hr/>
 @Body