facebook · sebmarkbage · May 1, 2025 · May 1, 2025
diff --git a/packages/react-dom-bindings/src/server/ReactFizzConfigDOM.js b/packages/react-dom-bindings/src/server/ReactFizzConfigDOM.js
@@ -5089,15 +5089,10 @@ export function writePreambleStart(
   destination: Destination,
   resumableState: ResumableState,
   renderState: RenderState,
-  willFlushAllSegments: boolean,
   skipExpect?: boolean, // Used as an override by ReactFizzConfigMarkup
 ): void {
   // This function must be called exactly once on every request
-  if (
-    enableFizzExternalRuntime &&
-    !willFlushAllSegments &&
-    renderState.externalRuntimeScript
-  ) {
+  if (enableFizzExternalRuntime && renderState.externalRuntimeScript) {
     // If the root segment is incomplete due to suspended tasks
     // (e.g. willFlushAllSegments = false) and we are using data
     // streaming format, ensure the external runtime is sent.

diff --git a/packages/react-dom/src/__tests__/ReactDOMFizzServer-test.js b/packages/react-dom/src/__tests__/ReactDOMFizzServer-test.js
@@ -3587,6 +3587,9 @@ describe('ReactDOMFizzServer', () => {
       '<script type="importmap">' +
         JSON.stringify(importMap) +
         '</script><script async="" src="foo"></script>' +
+        (gate(flags => flags.shouldUseFizzExternalRuntime)
+          ? '<script src="react-dom-bindings/src/server/ReactDOMServerExternalRuntime.js" async=""></script>'
+          : '') +
         '<link rel="expect" href="#«R»" blocking="render">',
     );
   });
@@ -4501,7 +4504,8 @@ describe('ReactDOMFizzServer', () => {
     expect(document.getElementsByTagName('script').length).toEqual(1);
   });
 
-  it('does not send the external runtime for static pages', async () => {
+  // @gate shouldUseFizzExternalRuntime
+  it('does (unfortunately) send the external runtime for static pages', async () => {
     await act(() => {
       const {pipe} = renderToPipeableStream(
         <html>
@@ -4515,11 +4519,11 @@ describe('ReactDOMFizzServer', () => {
     });
 
     // no scripts should be sent
-    expect(document.getElementsByTagName('script').length).toEqual(0);
+    expect(document.getElementsByTagName('script').length).toEqual(1);
 
     // the html should be as-is
     expect(document.documentElement.innerHTML).toEqual(
-      '<head><link rel="expect" href="#«R»" blocking="render"></head><body><p>hello world!</p><template id="«R»"></template></body>',
+      '<head><script src="react-dom-bindings/src/server/ReactDOMServerExternalRuntime.js" async=""></script><link rel="expect" href="#«R»" blocking="render"></head><body><p>hello world!</p><template id="«R»"></template></body>',
     );
   });
 
@@ -5317,7 +5321,9 @@ describe('ReactDOMFizzServer', () => {
       });
 
       expect(container.innerHTML).toEqual(
-        '<div>hello<b>world, <!-- -->Foo</b>!</div>',
+        (gate(flags => flags.shouldUseFizzExternalRuntime)
+          ? '<script src="react-dom-bindings/src/server/ReactDOMServerExternalRuntime.js" async=""></script>'
+          : '') + '<div>hello<b>world, <!-- -->Foo</b>!</div>',
       );
       const errors = [];
       ReactDOMClient.hydrateRoot(container, <App name="Foo" />, {
@@ -5518,7 +5524,7 @@ describe('ReactDOMFizzServer', () => {
         pipe(writable);
       });
 
-      expect(container.firstElementChild.outerHTML).toEqual(
+      expect(container.lastElementChild.outerHTML).toEqual(
         '<div>hello<b>world<!-- --></b></div>',
       );
 
@@ -5556,7 +5562,7 @@ describe('ReactDOMFizzServer', () => {
         pipe(writable);
       });
 
-      expect(container.firstElementChild.outerHTML).toEqual(
+      expect(container.lastElementChild.outerHTML).toEqual(
         '<div>hello<b>world</b></div>',
       );
 
@@ -5696,7 +5702,10 @@ describe('ReactDOMFizzServer', () => {
       });
 
       expect(container.innerHTML).toEqual(
-        '<div><!--$-->hello<!-- -->world<!-- --><!--/$--><!--$-->world<!-- --><!--/$--><!--$-->hello<!-- -->world<!-- --><br><!--/$--><!--$-->world<!-- --><br><!--/$--></div>',
+        (gate(flags => flags.shouldUseFizzExternalRuntime)
+          ? '<script src="react-dom-bindings/src/server/ReactDOMServerExternalRuntime.js" async=""></script>'
+          : '') +
+          '<div><!--$-->hello<!-- -->world<!-- --><!--/$--><!--$-->world<!-- --><!--/$--><!--$-->hello<!-- -->world<!-- --><br><!--/$--><!--$-->world<!-- --><br><!--/$--></div>',
       );
 
       const errors = [];
@@ -6499,7 +6508,11 @@ describe('ReactDOMFizzServer', () => {
     });
 
     expect(document.documentElement.outerHTML).toEqual(
-      '<html><head><link rel="expect" href="#«R»" blocking="render"></head><body><script>try { foo() } catch (e) {} ;</script><template id="«R»"></template></body></html>',
+      '<html><head>' +
+        (gate(flags => flags.shouldUseFizzExternalRuntime)
+          ? '<script src="react-dom-bindings/src/server/ReactDOMServerExternalRuntime.js" async=""></script>'
+          : '') +
+        '<link rel="expect" href="#«R»" blocking="render"></head><body><script>try { foo() } catch (e) {} ;</script><template id="«R»"></template></body></html>',
     );
   });
 

diff --git a/packages/react-dom/src/__tests__/ReactDOMFloat-test.js b/packages/react-dom/src/__tests__/ReactDOMFloat-test.js
@@ -701,6 +701,9 @@ describe('ReactDOMFloat', () => {
     });
     expect(chunks).toEqual([
       '<!DOCTYPE html><html><head><script async="" src="foo"></script>' +
+        (gate(flags => flags.shouldUseFizzExternalRuntime)
+          ? '<script src="react-dom/unstable_server-external-runtime" async=""></script>'
+          : '') +
         '<link rel="expect" href="#«R»" blocking="render"/><title>foo</title></head>' +
         '<body>bar<template id="«R»"></template>',
       '</body></html>',

diff --git a/packages/react-markup/src/ReactFizzConfigMarkup.js b/packages/react-markup/src/ReactFizzConfigMarkup.js
@@ -208,14 +208,12 @@ export function writePreambleStart(
   destination: Destination,
   resumableState: ResumableState,
   renderState: RenderState,
-  willFlushAllSegments: boolean,
   skipExpect?: boolean, // Used as an override by ReactFizzConfigMarkup
 ): void {
   return writePreambleStartImpl(
     destination,
     resumableState,
     renderState,
-    willFlushAllSegments,
     true, // skipExpect
   );
 }

diff --git a/packages/react-server/src/ReactFizzServer.js b/packages/react-server/src/ReactFizzServer.js
@@ -4835,14 +4835,7 @@ function flushPreamble(
   preambleSegments: Array<Array<Segment>>,
 ) {
   // The preamble is ready.
-  const willFlushAllSegments =
-    request.allPendingTasks === 0 && request.trackedPostpones === null;
-  writePreambleStart(
-    destination,
-    request.resumableState,
-    request.renderState,
-    willFlushAllSegments,
-  );
+  writePreambleStart(destination, request.resumableState, request.renderState);
   for (let i = 0; i < preambleSegments.length; i++) {
     const segments = preambleSegments[i];
     for (let j = 0; j < segments.length; j++) {