[Float][Fiber] Assume stylesheets in document are already loaded

When we made stylesheets suspend even during high priority updates we exposed a bug in the loading tracking of stylesheets that are loaded as part of the preamble. This allowed these stylesheets to put suspense boundaries into fallback mode more often than expected because cases where a stylesheet was server rendered could now cause a fallback to trigger which was never intended to happen.

This fix updates resource construction to evaluate whether the instance exists in the DOM prior to construction and if so marks the resource as loaded and inserted.

One ambiguity that needed to be solved still is how to tell whether a stylesheet rendered as part of a late Suspense boundary reveal is already loaded. I updated the instruction to clear out the loading promise after successfully loading. This is useful because later if we encounter this same resource again we can avoid the microtask if it is already loaded. It also means that we can concretely understand that if a stylesheet is in the DOM without this marker then it must have loaded (or errored) already.

Author: gnoff

packages/react-dom-bindings/src/client/ReactFiberConfigDOM.js

@@ -2412,17 +2412,30 @@ export function getResource(
         if (!resource) {
           // We asserted this above but Flow can't figure out that the type satisfies
           const ownerDocument = getDocumentFromRoot(resourceRoot);
-          resource = {
+          resource = ({
             type: 'stylesheet',
             instance: null,
             count: 0,
             state: {
               loading: NotLoaded,
               preload: null,
             },
-          };
+          }: StylesheetResource);
           styles.set(key, resource);
-          if (!preloadPropsMap.has(key)) {
+          const instance = ownerDocument.querySelector(
+            getStylesheetSelectorFromKey(key),
+          );
+          if (instance) {
+            const loadingState: ?Promise<mixed> = (instance: any)._p;
+            if (loadingState) {
+              // This instance is inserted as part of a boundary reveal and is not yet
+              // loaded
+            } else {
+              // This instance is already loaded
+              resource.instance = instance;
+              resource.state.loading = Loaded | Inserted;
+            }
+          } else if (!preloadPropsMap.has(key)) {
             preloadStylesheet(
               ownerDocument,
               key,
@@ -2601,26 +2614,21 @@ function preloadStylesheet(
 ) {
   preloadPropsMap.set(key, preloadProps);
 
-  if (!ownerDocument.querySelector(getStylesheetSelectorFromKey(key))) {
-    // There is no matching stylesheet instance in the Document.
-    // We will insert a preload now to kick off loading because
-    // we expect this stylesheet to commit
-    const preloadEl = ownerDocument.querySelector(
-      getPreloadStylesheetSelectorFromKey(key),
-    );
-    if (preloadEl) {
-      // If we find a preload already it was SSR'd and we won't have an actual
-      // loading state to track. For now we will just assume it is loaded
-      state.loading = Loaded;
-    } else {
-      const instance = ownerDocument.createElement('link');
-      state.preload = instance;
-      instance.addEventListener('load', () => (state.loading |= Loaded));
-      instance.addEventListener('error', () => (state.loading |= Errored));
-      setInitialProperties(instance, 'link', preloadProps);
-      markNodeAsHoistable(instance);
-      (ownerDocument.head: any).appendChild(instance);
-    }
+  const preloadEl = ownerDocument.querySelector(
+    getPreloadStylesheetSelectorFromKey(key),
+  );
+  if (preloadEl) {
+    // If we find a preload already it was SSR'd and we won't have an actual
+    // loading state to track. For now we will just assume it is loaded
+    state.loading = Loaded;
+  } else {
+    const instance = ownerDocument.createElement('link');
+    state.preload = instance;
+    instance.addEventListener('load', () => (state.loading |= Loaded));
+    instance.addEventListener('error', () => (state.loading |= Errored));
+    setInitialProperties(instance, 'link', preloadProps);
+    markNodeAsHoistable(instance);
+    (ownerDocument.head: any).appendChild(instance);
   }
 }
 

packages/react-dom-bindings/src/server/fizz-instruction-set/ReactDOMFizzInstructionSetExternalRuntime.js

@@ -80,18 +80,14 @@ export function completeBoundaryWithStyles(
           resourceEl.setAttribute(attr, stylesheetDescriptor[j++]);
         }
         loadingState = resourceEl['_p'] = new Promise((resolve, reject) => {
-          resourceEl.onload = resolve;
-          resourceEl.onerror = reject;
+          resourceEl.onload = cleanupWith.bind(resourceEl, resolve);
+          resourceEl.onerror = cleanupWith.bind(resourceEl, reject);
         });
         // Save this resource element so we can bailout if it is used again
         resourceMap.set(href, resourceEl);
       }
       media = resourceEl.getAttribute('media');
-      if (
-        loadingState &&
-        loadingState['s'] !== 'l' &&
-        (!media || window['matchMedia'](media).matches)
-      ) {
+      if (loadingState && (!media || window['matchMedia'](media).matches)) {
         dependencies.push(loadingState);
       }
       if (avoidInsert) {
@@ -138,4 +134,9 @@ export function completeBoundaryWithStyles(
   );
 }
 
+function cleanupWith(cb) {
+  delete this['_p'];
+  cb();
+}
+
 listenToFormSubmissionsForReplaying();

packages/react-dom-bindings/src/server/fizz-instruction-set/ReactDOMFizzInstructionSetInlineCodeStrings.js

@@ -82,18 +82,14 @@ export function completeBoundaryWithStyles(
           resourceEl.setAttribute(attr, stylesheetDescriptor[j++]);
         }
         loadingState = resourceEl['_p'] = new Promise((resolve, reject) => {
-          resourceEl.onload = resolve;
-          resourceEl.onerror = reject;
+          resourceEl.onload = cleanupWith.bind(resourceEl, resolve);
+          resourceEl.onerror = cleanupWith.bind(resourceEl, reject);
         });
         // Save this resource element so we can bailout if it is used again
         resourceMap.set(href, resourceEl);
       }
       media = resourceEl.getAttribute('media');
-      if (
-        loadingState &&
-        loadingState['s'] !== 'l' &&
-        (!media || window['matchMedia'](media).matches)
-      ) {
+      if (loadingState && (!media || window['matchMedia'](media).matches)) {
         dependencies.push(loadingState);
       }
       if (avoidInsert) {
@@ -139,3 +135,8 @@ export function completeBoundaryWithStyles(
     ),
   );
 }
+
+function cleanupWith(cb) {
+  delete this['_p'];
+  cb();
+}

packages/react-dom-bindings/src/server/fizz-instruction-set/ReactDOMFizzInstructionSetInlineSource.js

@@ -3348,6 +3348,46 @@ body {
     );
   });
 
+  it('will assume stylesheets already in the document have loaded if it cannot confirm it is not yet loaded', async () => {
+    await act(() => {
+      renderToPipeableStream(
+        <html>
+          <head>
+            <link rel="stylesheet" href="foo" data-precedence="default" />
+          </head>
+          <body>
+            <div id="foo" />
+          </body>
+        </html>,
+      ).pipe(writable);
+    });
+
+    const root = ReactDOMClient.createRoot(document.querySelector('#foo'));
+
+    root.render(
+      <div>
+        <Suspense fallback="loading...">
+          <link rel="stylesheet" href="foo" precedence="default" />
+          hello world
+        </Suspense>
+      </div>,
+    );
+
+    await waitForAll([]);
+    expect(getMeaningfulChildren(document)).toEqual(
+      <html>
+        <head>
+          <link rel="stylesheet" href="foo" data-precedence="default" />
+        </head>
+        <body>
+          <div id="foo">
+            <div>hello world</div>
+          </div>
+        </body>
+      </html>,
+    );
+  });
+
   it('can suspend commits on more than one root for the same resource at the same time', async () => {
     document.body.innerHTML = '';
     const container1 = document.createElement('div');