[flags] add enableEffectEventMutationPhase (#35548)

Small optimization for useEffectEvent. Not sure we even need a flag for
it, but it will be a nice killswitch.

As an added benefit, it fixes a bug when `enableViewTransition` is on,
where we were not updating the useEffectEvent callback when a tree went
from hidden to visible.

Author: rickhanlonii

packages/react-reconciler/src/ReactFiberCommitWork.js

@@ -47,6 +47,7 @@ import type {ViewTransitionState} from './ReactFiberViewTransitionComponent';
 import {
   alwaysThrottleRetries,
   enableCreateEventHandleAPI,
+  enableEffectEventMutationPhase,
   enableHiddenSubtreeInsertionEffectCleanup,
   enableProfilerTimer,
   enableProfilerCommitHooks,
@@ -499,7 +500,7 @@ function commitBeforeMutationEffectsOnFiber(
     case FunctionComponent:
     case ForwardRef:
     case SimpleMemoComponent: {
-      if ((flags & Update) !== NoFlags) {
+      if (!enableEffectEventMutationPhase && (flags & Update) !== NoFlags) {
         const updateQueue: FunctionComponentUpdateQueue | null =
           (finishedWork.updateQueue: any);
         const eventPayloads = updateQueue !== null ? updateQueue.events : null;
@@ -2042,6 +2043,24 @@ function commitMutationEffectsOnFiber(
     case ForwardRef:
     case MemoComponent:
     case SimpleMemoComponent: {
+      // Mutate event effect callbacks on the way down, before mutation effects.
+      // This ensures that parent event effects are mutated before child effects.
+      // This isn't a supported use case, so we can re-consider it,
+      // but this was the behavior we originally shipped.
+      if (enableEffectEventMutationPhase) {
+        if (flags & Update) {
+          const updateQueue: FunctionComponentUpdateQueue | null =
+            (finishedWork.updateQueue: any);
+          const eventPayloads =
+            updateQueue !== null ? updateQueue.events : null;
+          if (eventPayloads !== null) {
+            for (let ii = 0; ii < eventPayloads.length; ii++) {
+              const {ref, nextImpl} = eventPayloads[ii];
+              ref.impl = nextImpl;
+            }
+          }
+        }
+      }
       recursivelyTraverseMutationEffects(root, finishedWork, lanes);
       commitReconciliationEffects(finishedWork, lanes);
 

packages/react-reconciler/src/ReactFiberFlags.js

@@ -7,7 +7,10 @@
  * @flow
  */
 
-import {enableCreateEventHandleAPI} from 'shared/ReactFeatureFlags';
+import {
+  enableCreateEventHandleAPI,
+  enableEffectEventMutationPhase,
+} from 'shared/ReactFeatureFlags';
 
 export type Flags = number;
 
@@ -99,10 +102,11 @@ export const BeforeMutationMask: number =
       // TODO: Only need to visit Deletions during BeforeMutation phase if an
       // element is focused.
       Update | ChildDeletion | Visibility
-    : // TODO: The useEffectEvent hook uses the snapshot phase for clean up but it
-      // really should use the mutation phase for this or at least schedule an
-      // explicit Snapshot phase flag for this.
-      Update);
+    : // useEffectEvent uses the snapshot phase,
+      // but we're moving it to the mutation phase.
+      enableEffectEventMutationPhase
+      ? 0
+      : Update);
 
 // For View Transition support we use the snapshot phase to scan the tree for potentially
 // affected ViewTransition components.