Fix suspense fallback throttling (#24253)

* fix suspense throttling

* fix lint

* Tweak tests + another test

Co-authored-by: Dan Abramov <dan.abramov@gmail.com>

packages/react-reconciler/src/ReactFiberCommitWork.new.js

@@ -2209,17 +2209,22 @@ function commitMutationEffectsOnFiber(
       recursivelyTraverseMutationEffects(root, finishedWork, lanes);
       commitReconciliationEffects(finishedWork);
 
-      if (flags & Visibility) {
-        const newState: OffscreenState | null = finishedWork.memoizedState;
+      const offscreenFiber: Fiber = (finishedWork.child: any);
+
+      if (offscreenFiber.flags & Visibility) {
+        const newState: OffscreenState | null = offscreenFiber.memoizedState;
         const isHidden = newState !== null;
         if (isHidden) {
-          const wasHidden = current !== null && current.memoizedState !== null;
+          const wasHidden =
+            offscreenFiber.alternate !== null &&
+            offscreenFiber.alternate.memoizedState !== null;
           if (!wasHidden) {
             // TODO: Move to passive phase
             markCommitTimeOfFallback();
           }
         }
       }
+
       if (flags & Update) {
         try {
           commitSuspenseCallback(finishedWork);

packages/react-reconciler/src/ReactFiberCommitWork.old.js

@@ -2209,17 +2209,22 @@ function commitMutationEffectsOnFiber(
       recursivelyTraverseMutationEffects(root, finishedWork, lanes);
       commitReconciliationEffects(finishedWork);
 
-      if (flags & Visibility) {
-        const newState: OffscreenState | null = finishedWork.memoizedState;
+      const offscreenFiber: Fiber = (finishedWork.child: any);
+
+      if (offscreenFiber.flags & Visibility) {
+        const newState: OffscreenState | null = offscreenFiber.memoizedState;
         const isHidden = newState !== null;
         if (isHidden) {
-          const wasHidden = current !== null && current.memoizedState !== null;
+          const wasHidden =
+            offscreenFiber.alternate !== null &&
+            offscreenFiber.alternate.memoizedState !== null;
           if (!wasHidden) {
             // TODO: Move to passive phase
             markCommitTimeOfFallback();
           }
         }
       }
+
       if (flags & Update) {
         try {
           commitSuspenseCallback(finishedWork);

packages/react-reconciler/src/__tests__/ReactSuspense-test.internal.js

@@ -285,6 +285,117 @@ describe('ReactSuspense', () => {
     expect(root).toMatchRenderedOutput('AsyncAfter SuspenseSibling');
   });
 
+  it('throttles fallback committing globally', () => {
+    function Foo() {
+      Scheduler.unstable_yieldValue('Foo');
+      return (
+        <Suspense fallback={<Text text="Loading..." />}>
+          <AsyncText text="A" ms={200} />
+          <Suspense fallback={<Text text="Loading more..." />}>
+            <AsyncText text="B" ms={300} />
+          </Suspense>
+        </Suspense>
+      );
+    }
+
+    // Committing fallbacks should be throttled.
+    // First, advance some time to skip the first threshold.
+    jest.advanceTimersByTime(600);
+    Scheduler.unstable_advanceTime(600);
+
+    const root = ReactTestRenderer.create(<Foo />, {
+      unstable_isConcurrent: true,
+    });
+
+    expect(Scheduler).toFlushAndYield([
+      'Foo',
+      'Suspend! [A]',
+      'Suspend! [B]',
+      'Loading more...',
+      'Loading...',
+    ]);
+    expect(root).toMatchRenderedOutput('Loading...');
+
+    // Resolve A.
+    jest.advanceTimersByTime(200);
+    Scheduler.unstable_advanceTime(200);
+    expect(Scheduler).toHaveYielded(['Promise resolved [A]']);
+    expect(Scheduler).toFlushAndYield(['A', 'Suspend! [B]', 'Loading more...']);
+
+    // By this point, we have enough info to show "A" and "Loading more..."
+    // However, we've just shown the outer fallback. So we'll delay
+    // showing the inner fallback hoping that B will resolve soon enough.
+    expect(root).toMatchRenderedOutput('Loading...');
+
+    // Resolve B.
+    jest.advanceTimersByTime(100);
+    Scheduler.unstable_advanceTime(100);
+    expect(Scheduler).toHaveYielded(['Promise resolved [B]']);
+
+    // By this point, B has resolved.
+    // We're still showing the outer fallback.
+    expect(root).toMatchRenderedOutput('Loading...');
+    expect(Scheduler).toFlushAndYield(['A', 'B']);
+    // Then contents of both should pop in together.
+    expect(root).toMatchRenderedOutput('AB');
+  });
+
+  it('does not throttle fallback committing for too long', () => {
+    function Foo() {
+      Scheduler.unstable_yieldValue('Foo');
+      return (
+        <Suspense fallback={<Text text="Loading..." />}>
+          <AsyncText text="A" ms={200} />
+          <Suspense fallback={<Text text="Loading more..." />}>
+            <AsyncText text="B" ms={1200} />
+          </Suspense>
+        </Suspense>
+      );
+    }
+
+    // Committing fallbacks should be throttled.
+    // First, advance some time to skip the first threshold.
+    jest.advanceTimersByTime(600);
+    Scheduler.unstable_advanceTime(600);
+
+    const root = ReactTestRenderer.create(<Foo />, {
+      unstable_isConcurrent: true,
+    });
+
+    expect(Scheduler).toFlushAndYield([
+      'Foo',
+      'Suspend! [A]',
+      'Suspend! [B]',
+      'Loading more...',
+      'Loading...',
+    ]);
+    expect(root).toMatchRenderedOutput('Loading...');
+
+    // Resolve A.
+    jest.advanceTimersByTime(200);
+    Scheduler.unstable_advanceTime(200);
+    expect(Scheduler).toHaveYielded(['Promise resolved [A]']);
+    expect(Scheduler).toFlushAndYield(['A', 'Suspend! [B]', 'Loading more...']);
+
+    // By this point, we have enough info to show "A" and "Loading more..."
+    // However, we've just shown the outer fallback. So we'll delay
+    // showing the inner fallback hoping that B will resolve soon enough.
+    expect(root).toMatchRenderedOutput('Loading...');
+
+    // Wait some more. B is still not resolving.
+    jest.advanceTimersByTime(500);
+    Scheduler.unstable_advanceTime(500);
+    // Give up and render A with a spinner for B.
+    expect(root).toMatchRenderedOutput('ALoading more...');
+
+    // Resolve B.
+    jest.advanceTimersByTime(500);
+    Scheduler.unstable_advanceTime(500);
+    expect(Scheduler).toHaveYielded(['Promise resolved [B]']);
+    expect(Scheduler).toFlushAndYield(['B']);
+    expect(root).toMatchRenderedOutput('AB');
+  });
+
   // @gate !enableSyncDefaultUpdates
   it(
     'interrupts current render when something suspends with a ' +