Client render dehydrated Suspense boundaries on document load (#31620)

When streaming SSR while hydrating React will wait for Suspense
boundaries to be revealed by the SSR stream before attempting to hydrate
them. The rationale here is that the Server render is likely further
ahead of whatever the client would produce so waiting to let the server
stream in the UI is preferable to retrying on the client and possibly
delaying how quickly the primary content becomes available. However If
the connection closes early (user hits stop for instance) or there is a
server error which prevents additional HTML from being delivered to the
client this can put React into a broken state where the boundary never
resolves nor errors and the hydration never retries that boundary
freezing it in it's fallback state.

Once the document has fully loaded we know there is not way any
additional Suspense boundaries can arrive. This update changes react-dom
on the client to schedule client renders for any unfinished Suspense
boundaries upon document loading.

The technique for client rendering a fallback is pretty straight
forward. When hydrating a Suspense boundary if the Document is in
'complete' readyState we interpret pending boundaries as fallback
boundaries. If the readyState is not 'complete' we register an event to
retry the boundary when the DOMContentLoaded event fires.

To test this I needed JSDOM to model readyState. We previously had a
temporary implementation of readyState for SSR streaming but I ended up
implementing this as a mock of JSDOM that implements a fake readyState
that is mutable. It starts off in 'loading' readyState and you can
advance it by mutating document.readyState. You can also reset it to
'loading'. It fires events when changing states.

This seems like the least invasive way to get closer-to-real-browser
behavior in a way that won't require remembering this subtle detail
every time you create a test that asserts Suspense resolution order.

Author: gnoff

Diff for: packages/internal-test-utils/ReactJSDOM.js

@@ -0,0 +1,20 @@
+const JSDOMModule = jest.requireActual('jsdom');
+
+const OriginalJSDOM = JSDOMModule.JSDOM;
+
+module.exports = JSDOMModule;
+module.exports.JSDOM = function JSDOM() {
+  let result;
+  if (new.target) {
+    result = Reflect.construct(OriginalJSDOM, arguments);
+  } else {
+    result = JSDOM.apply(undefined, arguments);
+  }
+
+  require('./ReactJSDOMUtils').setupDocumentReadyState(
+    result.window.document,
+    result.window.Event,
+  );
+
+  return result;
+};

Diff for: packages/internal-test-utils/ReactJSDOMUtils.js

@@ -0,0 +1,33 @@
+export function setupDocumentReadyState(
+  document: Document,
+  Event: typeof Event,
+) {
+  let readyState: 0 | 1 | 2 = 0;
+  Object.defineProperty(document, 'readyState', {
+    get() {
+      switch (readyState) {
+        case 0:
+          return 'loading';
+        case 1:
+          return 'interactive';
+        case 2:
+          return 'complete';
+      }
+    },
+    set(value) {
+      if (value === 'interactive' && readyState < 1) {
+        readyState = 1;
+        document.dispatchEvent(new Event('readystatechange'));
+      } else if (value === 'complete' && readyState < 2) {
+        readyState = 2;
+        document.dispatchEvent(new Event('readystatechange'));
+        document.dispatchEvent(new Event('DOMContentLoaded'));
+      } else if (value === 'loading') {
+        // We allow resetting the readyState to loading mostly for pragamtism.
+        // tests that use this environment don't reset the document between tests.
+        readyState = 0;
+      }
+    },
+    configurable: true,
+  });
+}

Diff for: packages/react-dom-bindings/src/client/ReactFiberConfigDOM.js

@@ -194,6 +194,8 @@ const SUSPENSE_FALLBACK_START_DATA = '$!';
 const FORM_STATE_IS_MATCHING = 'F!';
 const FORM_STATE_IS_NOT_MATCHING = 'F';
 
+const DOCUMENT_READY_STATE_COMPLETE = 'complete';
+
 const STYLE = 'style';
 
 opaque type HostContextNamespace = 0 | 1 | 2;
@@ -1262,7 +1264,11 @@ export function isSuspenseInstancePending(instance: SuspenseInstance): boolean {
 export function isSuspenseInstanceFallback(
   instance: SuspenseInstance,
 ): boolean {
-  return instance.data === SUSPENSE_FALLBACK_START_DATA;
+  return (
+    instance.data === SUSPENSE_FALLBACK_START_DATA ||
+    (instance.data === SUSPENSE_PENDING_START_DATA &&
+      instance.ownerDocument.readyState === DOCUMENT_READY_STATE_COMPLETE)
+  );
 }
 
 export function getSuspenseInstanceFallbackErrorDetails(
@@ -1303,6 +1309,20 @@ export function registerSuspenseInstanceRetry(
   instance: SuspenseInstance,
   callback: () => void,
 ) {
+  const ownerDocument = instance.ownerDocument;
+  if (ownerDocument.readyState !== DOCUMENT_READY_STATE_COMPLETE) {
+    ownerDocument.addEventListener(
+      'DOMContentLoaded',
+      () => {
+        if (instance.data === SUSPENSE_PENDING_START_DATA) {
+          callback();
+        }
+      },
+      {
+        once: true,
+      },
+    );
+  }
   instance._reactRetry = callback;
 }