Don't flush synchronous work if we're in the middle of a ViewTransition async sequence (#32760)

Starting a View Transition is an async sequence. Since React can get a
sync update in the middle of sequence we sometimes interrupt that
sequence.

Currently, we don't actually cancel the View Transition so it can just
run as a partial. This ensures that we fully skip it when that happens,
as well as warn.

However, it's very easy to trigger this with just a setState in
useLayoutEffect right now. Therefore if we're inside the preparing
sequence of a startViewTransition, this delays work that would've
normally flushed in a microtask. ~Maybe we want to do the same for
Default work already scheduled through a scheduler Task.~ Edit: This was
already done.

`flushSync` currently will still lead to an interrupted View Transition
(with a warning). There's a tradeoff here whether we want to try our
best to preserve the guarantees of `flushSync` or favor the animation.
It's already possible to suspend at the root with `flushSync` which
means it's not always 100% guaranteed to commit anyway. We could treat
it as suspended. But let's see how much this is a problem in practice.

fixtures/view-transition/src/components/Page.js

@@ -2,6 +2,7 @@ import React, {
   unstable_ViewTransition as ViewTransition,
   unstable_Activity as Activity,
   unstable_useSwipeTransition as useSwipeTransition,
+  useLayoutEffect,
   useEffect,
   useState,
   useId,
@@ -68,6 +69,16 @@ export default function Page({url, navigate}) {
     return () => clearInterval(timer);
   }, []);
 
+  useLayoutEffect(() => {
+    // Calling a default update should not interrupt ViewTransitions but
+    // a flushSync will.
+    // Promise.resolve().then(() => {
+    //   flushSync(() => {
+    setCounter(c => c + 10);
+    //  });
+    // });
+  }, [show]);
+
   const exclamation = (
     <ViewTransition name="exclamation" onShare={onTransition}>
       <span>!</span>

packages/react-art/src/ReactFiberConfigART.js

@@ -538,14 +538,16 @@ export function hasInstanceAffectedParent(
 }
 
 export function startViewTransition() {
-  return false;
+  return null;
 }
 
-export type RunningGestureTransition = null;
+export type RunningViewTransition = null;
 
-export function startGestureTransition() {}
+export function startGestureTransition() {
+  return null;
+}
 
-export function stopGestureTransition(transition: RunningGestureTransition) {}
+export function stopViewTransition(transition: RunningViewTransition) {}
 
 export type ViewTransitionInstance = null | {name: string, ...};
 

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

@@ -1687,7 +1687,7 @@ export function startViewTransition(
   spawnedWorkCallback: () => void,
   passiveCallback: () => mixed,
   errorCallback: mixed => void,
-): boolean {
+): null | RunningViewTransition {
   const ownerDocument: Document =
     rootContainer.nodeType === DOCUMENT_NODE
       ? (rootContainer: any)
@@ -1764,7 +1764,7 @@ export function startViewTransition(
       }
       passiveCallback();
     });
-    return true;
+    return transition;
   } catch (x) {
     // We use the error as feature detection.
     // The only thing that should throw is if startViewTransition is missing
@@ -1772,11 +1772,17 @@ export function startViewTransition(
     // I.e. it's before the View Transitions v2 spec. We only support View
     // Transitions v2 otherwise we fallback to not animating to ensure that
     // we're not animating with the wrong animation mapped.
-    return false;
+    // Flush remaining work synchronously.
+    mutationCallback();
+    layoutCallback();
+    // Skip afterMutationCallback(). We don't need it since we're not animating.
+    spawnedWorkCallback();
+    // Skip passiveCallback(). Spawned work will schedule a task.
+    return null;
   }
 }
 
-export type RunningGestureTransition = {
+export type RunningViewTransition = {
   skipTransition(): void,
   ...
 };
@@ -1900,7 +1906,7 @@ export function startGestureTransition(
   mutationCallback: () => void,
   animateCallback: () => void,
   errorCallback: mixed => void,
-): null | RunningGestureTransition {
+): null | RunningViewTransition {
   const ownerDocument: Document =
     rootContainer.nodeType === DOCUMENT_NODE
       ? (rootContainer: any)
@@ -2072,7 +2078,7 @@ export function startGestureTransition(
   }
 }
 
-export function stopGestureTransition(transition: RunningGestureTransition) {
+export function stopViewTransition(transition: RunningViewTransition) {
   transition.skipTransition();
 }
 

packages/react-native-renderer/src/ReactFiberConfigNative.js

@@ -653,11 +653,16 @@ export function startViewTransition(
   spawnedWorkCallback: () => void,
   passiveCallback: () => mixed,
   errorCallback: mixed => void,
-): boolean {
-  return false;
+): null | RunningViewTransition {
+  mutationCallback();
+  layoutCallback();
+  // Skip afterMutationCallback(). We don't need it since we're not animating.
+  spawnedWorkCallback();
+  // Skip passiveCallback(). Spawned work will schedule a task.
+  return null;
 }
 
-export type RunningGestureTransition = null;
+export type RunningViewTransition = null;
 
 export function startGestureTransition(
   rootContainer: Container,
@@ -668,13 +673,13 @@ export function startGestureTransition(
   mutationCallback: () => void,
   animateCallback: () => void,
   errorCallback: mixed => void,
-): RunningGestureTransition {
+): null | RunningViewTransition {
   mutationCallback();
   animateCallback();
   return null;
 }
 
-export function stopGestureTransition(transition: RunningGestureTransition) {}
+export function stopViewTransition(transition: RunningViewTransition) {}
 
 export type ViewTransitionInstance = null | {name: string, ...};
 

packages/react-noop-renderer/src/createReactNoop.js

@@ -93,7 +93,7 @@ export type TransitionStatus = mixed;
 
 export type FormInstance = Instance;
 
-export type RunningGestureTransition = null;
+export type RunningViewTransition = null;
 
 export type ViewTransitionInstance = null | {name: string, ...};
 
@@ -826,12 +826,18 @@ function createReactNoop(reconciler: Function, useMutation: boolean) {
           rootContainer: Container,
           transitionTypes: null | TransitionTypes,
           mutationCallback: () => void,
-          afterMutationCallback: () => void,
           layoutCallback: () => void,
+          afterMutationCallback: () => void,
+          spawnedWorkCallback: () => void,
           passiveCallback: () => mixed,
           errorCallback: mixed => void,
-        ): boolean {
-          return false;
+        ): null | RunningViewTransition {
+          mutationCallback();
+          layoutCallback();
+          // Skip afterMutationCallback(). We don't need it since we're not animating.
+          spawnedWorkCallback();
+          // Skip passiveCallback(). Spawned work will schedule a task.
+          return null;
         },
 
         startGestureTransition(
@@ -843,13 +849,13 @@ function createReactNoop(reconciler: Function, useMutation: boolean) {
           mutationCallback: () => void,
           animateCallback: () => void,
           errorCallback: mixed => void,
-        ): RunningGestureTransition {
+        ): null | RunningViewTransition {
           mutationCallback();
           animateCallback();
           return null;
         },
 
-        stopGestureTransition(transition: RunningGestureTransition) {},
+        stopViewTransition(transition: RunningViewTransition) {},
 
         createViewTransitionInstance(name: string): ViewTransitionInstance {
           return null;

packages/react-reconciler/src/ReactFiberConfigWithNoMutation.js

@@ -51,9 +51,9 @@ export const wasInstanceInViewport = shim;
 export const hasInstanceChanged = shim;
 export const hasInstanceAffectedParent = shim;
 export const startViewTransition = shim;
-export type RunningGestureTransition = null;
+export type RunningViewTransition = null;
 export const startGestureTransition = shim;
-export const stopGestureTransition = shim;
+export const stopViewTransition = shim;
 export type ViewTransitionInstance = null | {name: string, ...};
 export const createViewTransitionInstance = shim;
 export type GestureTimeline = any;

packages/react-reconciler/src/ReactFiberGestureScheduler.js

@@ -8,10 +8,7 @@
  */
 
 import type {FiberRoot} from './ReactInternalTypes';
-import type {
-  GestureTimeline,
-  RunningGestureTransition,
-} from './ReactFiberConfig';
+import type {GestureTimeline, RunningViewTransition} from './ReactFiberConfig';
 
 import {
   GestureLane,
@@ -21,7 +18,7 @@ import {
 import {ensureRootIsScheduled} from './ReactFiberRootScheduler';
 import {
   subscribeToGestureDirection,
-  stopGestureTransition,
+  stopViewTransition,
 } from './ReactFiberConfig';
 
 // This type keeps track of any scheduled or active gestures.
@@ -33,7 +30,7 @@ export type ScheduledGesture = {
   rangeCurrent: number, // The starting offset along the timeline.
   rangeNext: number, // The end along the timeline where the next state is reached.
   cancel: () => void, // Cancel the subscription to direction change.
-  running: null | RunningGestureTransition, // Used to cancel the running transition after we're done.
+  running: null | RunningViewTransition, // Used to cancel the running transition after we're done.
   prev: null | ScheduledGesture, // The previous scheduled gesture in the queue for this root.
   next: null | ScheduledGesture, // The next scheduled gesture in the queue for this root.
 };
@@ -144,7 +141,7 @@ export function cancelScheduledGesture(
       } else {
         gesture.running = null;
         // If there's no work scheduled so we can stop the View Transition right away.
-        stopGestureTransition(runningTransition);
+        stopViewTransition(runningTransition);
       }
     }
   }
@@ -183,7 +180,7 @@ export function stopCompletedGestures(root: FiberRoot) {
   root.stoppingGestures = null;
   while (gesture !== null) {
     if (gesture.running !== null) {
-      stopGestureTransition(gesture.running);
+      stopViewTransition(gesture.running);
       gesture.running = null;
     }
     const nextGesture = gesture.next;

packages/react-reconciler/src/ReactFiberRootScheduler.js

@@ -310,7 +310,12 @@ function processRootScheduleInMicrotask() {
 
   // At the end of the microtask, flush any pending synchronous work. This has
   // to come at the end, because it does actual rendering work that might throw.
-  flushSyncWorkAcrossRoots_impl(syncTransitionLanes, false);
+  // If we're in the middle of a View Transition async sequence, we don't want to
+  // interrupt that sequence. Instead, we'll flush any remaining work when it
+  // completes.
+  if (!hasPendingCommitEffects()) {
+    flushSyncWorkAcrossRoots_impl(syncTransitionLanes, false);
+  }
 }
 
 function scheduleTaskForRootDuringMicrotask(

packages/react-reconciler/src/ReactFiberWorkLoop.js

@@ -21,7 +21,11 @@ import type {
   TransitionAbort,
 } from './ReactFiberTracingMarkerComponent';
 import type {OffscreenInstance} from './ReactFiberActivityComponent';
-import type {Resource, ViewTransitionInstance} from './ReactFiberConfig';
+import type {
+  Resource,
+  ViewTransitionInstance,
+  RunningViewTransition,
+} from './ReactFiberConfig';
 import type {RootState} from './ReactFiberRoot';
 import {
   getViewTransitionName,
@@ -102,6 +106,7 @@ import {
   trackSchedulerEvent,
   startViewTransition,
   startGestureTransition,
+  stopViewTransition,
   createViewTransitionInstance,
 } from './ReactFiberConfig';
 
@@ -665,6 +670,7 @@ let pendingEffectsRemainingLanes: Lanes = NoLanes;
 let pendingEffectsRenderEndTime: number = -0; // Profiling-only
 let pendingPassiveTransitions: Array<Transition> | null = null;
 let pendingRecoverableErrors: null | Array<CapturedValue<mixed>> = null;
+let pendingViewTransition: null | RunningViewTransition = null;
 let pendingViewTransitionEvents: Array<(types: Array<string>) => void> | null =
   null;
 let pendingTransitionTypes: null | TransitionTypes = null;
@@ -3503,10 +3509,8 @@ function commitRoot(
   }
 
   pendingEffectsStatus = PENDING_MUTATION_PHASE;
-  const startedViewTransition =
-    enableViewTransition &&
-    willStartViewTransition &&
-    startViewTransition(
+  if (enableViewTransition && willStartViewTransition) {
+    pendingViewTransition = startViewTransition(
       root.containerInfo,
       pendingTransitionTypes,
       flushMutationEffects,
@@ -3516,7 +3520,7 @@ function commitRoot(
       flushPassiveEffects,
       reportViewTransitionError,
     );
-  if (!startedViewTransition) {
+  } else {
     // Flush synchronously.
     flushMutationEffects();
     flushLayoutEffects();
@@ -3646,6 +3650,8 @@ function flushSpawnedWork(): void {
   }
   pendingEffectsStatus = NO_PENDING_EFFECTS;
 
+  pendingViewTransition = null; // The view transition has now fully started.
+
   // Tell Scheduler to yield at the end of the frame, so the browser has an
   // opportunity to paint.
   requestPaint();
@@ -3915,7 +3921,7 @@ function commitGestureOnRoot(
   pendingTransitionTypes = null;
   pendingEffectsStatus = PENDING_GESTURE_MUTATION_PHASE;
 
-  finishedGesture.running = startGestureTransition(
+  pendingViewTransition = finishedGesture.running = startGestureTransition(
     root.containerInfo,
     finishedGesture.provider,
     finishedGesture.rangeCurrent,
@@ -3975,6 +3981,8 @@ function flushGestureAnimations(): void {
   pendingFinishedWork = (null: any); // Clear for GC purposes.
   pendingEffectsLanes = NoLanes;
 
+  pendingViewTransition = null; // The view transition has now fully started.
+
   const prevTransition = ReactSharedInternals.T;
   ReactSharedInternals.T = null;
   const previousPriority = getCurrentUpdatePriority();
@@ -4025,8 +4033,27 @@ function releaseRootPooledCache(root: FiberRoot, remainingLanes: Lanes) {
   }
 }
 
+let didWarnAboutInterruptedViewTransitions = false;
+
 export function flushPendingEffects(wasDelayedCommit?: boolean): boolean {
   // Returns whether passive effects were flushed.
+  if (enableViewTransition && pendingViewTransition !== null) {
+    // If we forced a flush before the View Transition full started then we skip it.
+    // This ensures that we're not running a partial animation.
+    stopViewTransition(pendingViewTransition);
+    if (__DEV__) {
+      if (!didWarnAboutInterruptedViewTransitions) {
+        didWarnAboutInterruptedViewTransitions = true;
+        console.warn(
+          'A flushSync update cancelled a View Transition because it was called ' +
+            'while the View Transition was still preparing. To preserve the synchronous ' +
+            'semantics, React had to skip the View Transition. If you can, try to avoid ' +
+            "flushSync() in a scenario that's likely to interfere.",
+        );
+      }
+    }
+    pendingViewTransition = null;
+  }
   flushGestureMutations();
   flushGestureAnimations();
   flushMutationEffects();

packages/react-reconciler/src/forks/ReactFiberConfig.custom.js

@@ -40,7 +40,7 @@ export opaque type NoTimeout = mixed;
 export opaque type RendererInspectionConfig = mixed;
 export opaque type TransitionStatus = mixed;
 export opaque type FormInstance = mixed;
-export type RunningGestureTransition = mixed;
+export type RunningViewTransition = mixed;
 export type ViewTransitionInstance = null | {name: string, ...};
 export opaque type InstanceMeasurement = mixed;
 export type EventResponder = any;
@@ -155,7 +155,7 @@ export const hasInstanceChanged = $$$config.hasInstanceChanged;
 export const hasInstanceAffectedParent = $$$config.hasInstanceAffectedParent;
 export const startViewTransition = $$$config.startViewTransition;
 export const startGestureTransition = $$$config.startGestureTransition;
-export const stopGestureTransition = $$$config.stopGestureTransition;
+export const stopViewTransition = $$$config.stopViewTransition;
 export const getCurrentGestureOffset = $$$config.getCurrentGestureOffset;
 export const subscribeToGestureDirection =
   $$$config.subscribeToGestureDirection;

packages/react-test-renderer/src/ReactFiberConfigTestHost.js

@@ -422,11 +422,16 @@ export function startViewTransition(
   spawnedWorkCallback: () => void,
   passiveCallback: () => mixed,
   errorCallback: mixed => void,
-): boolean {
-  return false;
+): null | RunningViewTransition {
+  mutationCallback();
+  layoutCallback();
+  // Skip afterMutationCallback(). We don't need it since we're not animating.
+  spawnedWorkCallback();
+  // Skip passiveCallback(). Spawned work will schedule a task.
+  return null;
 }
 
-export type RunningGestureTransition = null;
+export type RunningViewTransition = null;
 
 export function startGestureTransition(
   rootContainer: Container,
@@ -437,13 +442,13 @@ export function startGestureTransition(
   mutationCallback: () => void,
   animateCallback: () => void,
   errorCallback: mixed => void,
-): RunningGestureTransition {
+): null | RunningViewTransition {
   mutationCallback();
   animateCallback();
   return null;
 }
 
-export function stopGestureTransition(transition: RunningGestureTransition) {}
+export function stopViewTransition(transition: RunningViewTransition) {}
 
 export type ViewTransitionInstance = null | {name: string, ...};