Fix: uDV initialValue suspends without switching to final

Fixes a bug in the experimental `initialValue` option for
`useDeferredValue` (added in #27500).

If rendering the `initialValue` causes the tree to suspend, React should
skip it and switch to rendering the final value instead. It should not
wait for `initialValue` to resolve.

This is not just an optimization, because in some cases the initial
value may _never_ resolve — intentionally. For example, if the
application does not provide an instant fallback state. This capability
is, in fact, the primary motivation for the `initialValue` API.

I mostly implemented this correctly in the original PR, but I missed
some cases where it wasn't working:

- If there's no Suspense boundary between the `useDeferredValue` hook
and the component that suspends, and we're not in the shell of the
transition (i.e. there's a parent Suspense boundary wrapping the
`useDeferredValue` hook), the deferred task would get
incorrectly dropped.
- Similarly, if there's no Suspense boundary between the
`useDeferredValue` hook and the component that suspends, and we're
rendering a synchronous update, the deferred task would get incorrectly
dropped.

What these cases have in common is that it causes the `useDeferredValue`
hook itself to be replaced by a Suspense fallback. The fix was the same
for both. (It already worked in cases where there's no Suspense
fallback at all, because those are handled differently, at the root.)

The way I discovered this was when investigating a particular bug in
Next.js that would happen during a 'popstate' transition (back/forward),
but not during a regular navigation. That's because we render popstate
transitions synchronously to preserve browser's scroll position — which
in this case triggered the second scenario above.

packages/react-dom/src/__tests__/ReactDOMFizzDeferredValue-test.js

@@ -85,6 +85,38 @@ describe('ReactDOMFizzForm', () => {
     expect(container.textContent).toEqual('Final');
   });
 
+  // @gate enableUseDeferredValueInitialArg
+  // @gate enablePostpone
+  it(
+    'if initial value postpones during hydration, it will switch to the ' +
+      'final value instead',
+    async () => {
+      function Content() {
+        const isInitial = useDeferredValue(false, true);
+        if (isInitial) {
+          React.unstable_postpone();
+        }
+        return <Text text="Final" />;
+      }
+
+      function App() {
+        return (
+          <Suspense fallback={<Text text="Loading..." />}>
+            <Content />
+          </Suspense>
+        );
+      }
+
+      const stream = await ReactDOMServer.renderToReadableStream(<App />);
+      await readIntoContainer(stream);
+      expect(container.textContent).toEqual('Loading...');
+
+      // After hydration, it's updated to the final value
+      await act(() => ReactDOMClient.hydrateRoot(container, <App />));
+      expect(container.textContent).toEqual('Final');
+    },
+  );
+
   // @gate enableUseDeferredValueInitialArg
   it(
     'useDeferredValue during hydration has higher priority than remaining ' +

packages/react-reconciler/src/ReactFiberBeginWork.js

@@ -90,6 +90,7 @@ import {
   ShouldCapture,
   ForceClientRender,
   Passive,
+  DidDefer,
 } from './ReactFiberFlags';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
 import {
@@ -257,6 +258,7 @@ import {
   renderDidSuspendDelayIfPossible,
   markSkippedUpdateLanes,
   getWorkInProgressRoot,
+  peekDeferredLane,
 } from './ReactFiberWorkLoop';
 import {enqueueConcurrentRenderForLane} from './ReactFiberConcurrentUpdates';
 import {pushCacheProvider, CacheContext} from './ReactFiberCacheComponent';
@@ -2228,9 +2230,22 @@ function shouldRemainOnFallback(
   );
 }
 
-function getRemainingWorkInPrimaryTree(current: Fiber, renderLanes: Lanes) {
-  // TODO: Should not remove render lanes that were pinged during this render
-  return removeLanes(current.childLanes, renderLanes);
+function getRemainingWorkInPrimaryTree(
+  current: Fiber | null,
+  primaryTreeDidDefer: boolean,
+  renderLanes: Lanes,
+) {
+  let remainingLanes =
+    current !== null ? removeLanes(current.childLanes, renderLanes) : NoLanes;
+  if (primaryTreeDidDefer) {
+    // A useDeferredValue hook spawned a deferred task inside the primary tree.
+    // Ensure that we retry this component at the deferred priority.
+    // TODO: We could make this a per-subtree value instead of a global one.
+    // Would need to track it on the context stack somehow, similar to what
+    // we'd have to do for resumable contexts.
+    remainingLanes = mergeLanes(remainingLanes, peekDeferredLane());
+  }
+  return remainingLanes;
 }
 
 function updateSuspenseComponent(
@@ -2259,6 +2274,11 @@ function updateSuspenseComponent(
     workInProgress.flags &= ~DidCapture;
   }
 
+  // Check if the primary children spawned a deferred task (useDeferredValue)
+  // during the first pass.
+  const didPrimaryChildrenDefer = (workInProgress.flags & DidDefer) !== NoFlags;
+  workInProgress.flags &= ~DidDefer;
+
   // OK, the next part is confusing. We're about to reconcile the Suspense
   // boundary's children. This involves some custom reconciliation logic. Two
   // main reasons this is so complicated.
@@ -2329,6 +2349,11 @@ function updateSuspenseComponent(
       const primaryChildFragment: Fiber = (workInProgress.child: any);
       primaryChildFragment.memoizedState =
         mountSuspenseOffscreenState(renderLanes);
+      primaryChildFragment.childLanes = getRemainingWorkInPrimaryTree(
+        current,
+        didPrimaryChildrenDefer,
+        renderLanes,
+      );
       workInProgress.memoizedState = SUSPENDED_MARKER;
       if (enableTransitionTracing) {
         const currentTransitions = getPendingTransitions();
@@ -2368,6 +2393,11 @@ function updateSuspenseComponent(
       const primaryChildFragment: Fiber = (workInProgress.child: any);
       primaryChildFragment.memoizedState =
         mountSuspenseOffscreenState(renderLanes);
+      primaryChildFragment.childLanes = getRemainingWorkInPrimaryTree(
+        current,
+        didPrimaryChildrenDefer,
+        renderLanes,
+      );
       workInProgress.memoizedState = SUSPENDED_MARKER;
 
       // TODO: Transition Tracing is not yet implemented for CPU Suspense.
@@ -2402,6 +2432,7 @@ function updateSuspenseComponent(
           current,
           workInProgress,
           didSuspend,
+          didPrimaryChildrenDefer,
           nextProps,
           dehydrated,
           prevState,
@@ -2464,6 +2495,7 @@ function updateSuspenseComponent(
       }
       primaryChildFragment.childLanes = getRemainingWorkInPrimaryTree(
         current,
+        didPrimaryChildrenDefer,
         renderLanes,
       );
       workInProgress.memoizedState = SUSPENDED_MARKER;
@@ -2834,6 +2866,7 @@ function updateDehydratedSuspenseComponent(
   current: Fiber,
   workInProgress: Fiber,
   didSuspend: boolean,
+  didPrimaryChildrenDefer: boolean,
   nextProps: any,
   suspenseInstance: SuspenseInstance,
   suspenseState: SuspenseState,
@@ -3063,6 +3096,11 @@ function updateDehydratedSuspenseComponent(
       const primaryChildFragment: Fiber = (workInProgress.child: any);
       primaryChildFragment.memoizedState =
         mountSuspenseOffscreenState(renderLanes);
+      primaryChildFragment.childLanes = getRemainingWorkInPrimaryTree(
+        current,
+        didPrimaryChildrenDefer,
+        renderLanes,
+      );
       workInProgress.memoizedState = SUSPENDED_MARKER;
       return fallbackChildFragment;
     }

packages/react-reconciler/src/ReactFiberFlags.js

@@ -41,6 +41,7 @@ export const StoreConsistency = /*             */ 0b0000000000000100000000000000
 // possible, because we're about to run out of bits.
 export const ScheduleRetry = StoreConsistency;
 export const ShouldSuspendCommit = Visibility;
+export const DidDefer = ContentReset;
 
 export const LifecycleEffectMask =
   Passive | Update | Callback | Ref | Snapshot | StoreConsistency;

packages/react-reconciler/src/ReactFiberWorkLoop.js

@@ -127,6 +127,7 @@ import {
   Visibility,
   MountPassiveDev,
   MountLayoutDev,
+  DidDefer,
 } from './ReactFiberFlags';
 import {
   NoLanes,
@@ -714,6 +715,20 @@ export function requestDeferredLane(): Lane {
       workInProgressDeferredLane = requestTransitionLane();
     }
   }
+
+  // Mark the parent Suspense boundary so it knows to spawn the deferred lane.
+  const suspenseHandler = getSuspenseHandler();
+  if (suspenseHandler !== null) {
+    // TODO: As an optimization, we shouldn't entangle the lanes at the root; we
+    // can entangle them using the baseLanes of the Suspense boundary instead.
+    // We only need to do something special if there's no Suspense boundary.
+    suspenseHandler.flags |= DidDefer;
+  }
+
+  return workInProgressDeferredLane;
+}
+
+export function peekDeferredLane(): Lane {
   return workInProgressDeferredLane;
 }
 
@@ -1361,7 +1376,7 @@ export function performSyncWorkOnRoot(root: FiberRoot, lanes: Lanes): null {
     // The render unwound without completing the tree. This happens in special
     // cases where need to exit the current render without producing a
     // consistent tree or committing.
-    markRootSuspended(root, lanes, NoLane);
+    markRootSuspended(root, lanes, workInProgressDeferredLane);
     ensureRootIsScheduled(root);
     return null;
   }

packages/react-reconciler/src/__tests__/ReactDeferredValue-test.js

@@ -409,7 +409,7 @@ describe('ReactDeferredValue', () => {
   // @gate enableUseDeferredValueInitialArg
   it(
     'if a suspended render spawns a deferred task, we can switch to the ' +
-      'deferred task without finishing the original one',
+      'deferred task without finishing the original one (no Suspense boundary)',
     async () => {
       function App() {
         const text = useDeferredValue('Final', 'Loading...');
@@ -439,6 +439,87 @@ describe('ReactDeferredValue', () => {
     },
   );
 
+  // @gate enableUseDeferredValueInitialArg
+  it(
+    'if a suspended render spawns a deferred task, we can switch to the ' +
+      'deferred task without finishing the original one (no Suspense boundary, ' +
+      'synchronous parent update)',
+    async () => {
+      function App() {
+        const text = useDeferredValue('Final', 'Loading...');
+        return <AsyncText text={text} />;
+      }
+
+      const root = ReactNoop.createRoot();
+      // TODO: This made me realize that we don't warn if an update spawns a
+      // deferred task without being wrapped with `act`. Usually it would work
+      // anyway because the parent task has to wrapped with `act`... but not
+      // if it was flushed with `flushSync` instead.
+      await act(() => {
+        ReactNoop.flushSync(() => root.render(<App />));
+      });
+      assertLog([
+        'Suspend! [Loading...]',
+        // The initial value suspended, so we attempt the final value, which
+        // also suspends.
+        'Suspend! [Final]',
+      ]);
+      expect(root).toMatchRenderedOutput(null);
+
+      // The final value loads, so we can skip the initial value entirely.
+      await act(() => resolveText('Final'));
+      assertLog(['Final']);
+      expect(root).toMatchRenderedOutput('Final');
+
+      // When the initial value finally loads, nothing happens because we no
+      // longer need it.
+      await act(() => resolveText('Loading...'));
+      assertLog([]);
+      expect(root).toMatchRenderedOutput('Final');
+    },
+  );
+
+  // @gate enableUseDeferredValueInitialArg
+  it(
+    'if a suspended render spawns a deferred task, we can switch to the ' +
+      'deferred task without finishing the original one (Suspense boundary)',
+    async () => {
+      function App() {
+        const text = useDeferredValue('Final', 'Loading...');
+        return <AsyncText text={text} />;
+      }
+
+      const root = ReactNoop.createRoot();
+      await act(() =>
+        root.render(
+          <Suspense fallback={<Text text="Fallback" />}>
+            <App />
+          </Suspense>,
+        ),
+      );
+      assertLog([
+        'Suspend! [Loading...]',
+        'Fallback',
+
+        // The initial value suspended, so we attempt the final value, which
+        // also suspends.
+        'Suspend! [Final]',
+      ]);
+      expect(root).toMatchRenderedOutput('Fallback');
+
+      // The final value loads, so we can skip the initial value entirely.
+      await act(() => resolveText('Final'));
+      assertLog(['Final']);
+      expect(root).toMatchRenderedOutput('Final');
+
+      // When the initial value finally loads, nothing happens because we no
+      // longer need it.
+      await act(() => resolveText('Loading...'));
+      assertLog([]);
+      expect(root).toMatchRenderedOutput('Final');
+    },
+  );
+
   // @gate enableUseDeferredValueInitialArg
   it(
     'if a suspended render spawns a deferred task that also suspends, we can ' +