[internal] add feature flag to fix event handle API in Activity

[rickhanlonii]

Noticed this TODO was never implemented, so this implements support for Activity inside the deprecated, never shipping enableCreateEventHandleAPI feature used internally until we remove the flag.

Also adds a bunch of test for this API with Activity and LegacyHidden, with inline gates for the differences with the flags.

Below is the claude summary.

Details

Fix Event Handle API blur events for Activity

Problem

The beforeblur and afterblur events from the createEventHandle API were not firing when a focused element was hidden using <React.Activity>. This was because the blur check in commitBeforeMutationEffectsOnFiber only handled SuspenseComponent, not OffscreenComponent (which Activity uses internally).

Solution

Added a check for OffscreenComponent visibility transitions behind a new feature flag enableEventAPIActivityFix.

---

Files Modified

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

Added blur event handling for Activity (OffscreenComponent):

// Check if an OffscreenComponent (Activity) is being hidden with focus inside.
if (enableEventAPIActivityFix) {
  if (
    finishedWork.tag === OffscreenComponent &&
    current !== null &&
    current.memoizedState === null && // was visible
    finishedWork.memoizedState !== null && // now hidden
    doesFiberContain(finishedWork, focusedInstanceHandle)
  ) {
    shouldFireAfterActiveInstanceBlur = true;
    beforeActiveInstanceBlur(finishedWork);
  }
}

2. Feature Flag Files

Added enableEventAPIActivityFix to all feature flag files:

File	Value
ReactFeatureFlags.js	false (default)
ReactFeatureFlags.www-dynamic.js	__VARIANT__ (dynamic)
ReactFeatureFlags.www.js	imports from dynamic
ReactFeatureFlags.test-renderer.js	false
ReactFeatureFlags.test-renderer.www.js	false
ReactFeatureFlags.test-renderer.native-fb.js	false

3. packages/react-dom/src/events/__tests__/DOMPluginEventSystem-test.internal.js

Added 22 new tests (from 120 → 142 total):

Activity tests (6 tests)

• beforeblur and afterblur are called after a focused element is hidden
• beforeblur and afterblur are called after a nested focused element is hidden
• beforeblur should skip handlers from a hidden subtree after the focused element is hidden via Activity

LegacyHidden tests (6 tests)

• beforeblur and afterblur are not called after a focused element is hidden inside LegacyHidden
• beforeblur and afterblur are not called after a nested focused element is hidden inside LegacyHidden
• beforeblur is not called after the focused element is hidden inside LegacyHidden

Nested Activity + LegacyHidden tests (10 tests)

Nesting	Scenario	Expected
<Activity><LegacyHidden>	Hide outer (Activity)	blur fires
<Activity><LegacyHidden>	Hide inner (LegacyHidden)	no blur
<Activity><LegacyHidden>	Hide both simultaneously	blur fires once
<Activity><LegacyHidden>	Hide LegacyHidden, then Activity	blur on Activity
<Activity><LegacyHidden>	Hide Activity, then LegacyHidden	blur on Activity, no second blur
<LegacyHidden><Activity>	Hide outer (LegacyHidden)	no blur
<LegacyHidden><Activity>	Hide inner (Activity)	blur fires
<LegacyHidden><Activity>	Hide both simultaneously	blur fires once
<LegacyHidden><Activity>	Hide LegacyHidden, then Activity	blur on Activity
<LegacyHidden><Activity>	Hide Activity, then LegacyHidden	blur on Activity, no second blur

---

Key Design Decisions

1. Activity triggers blur, LegacyHidden does not - LegacyHidden behavior remains unchanged for backwards compatibility

2. Inline gate pattern - Tests use if(gate('enableEventAPIActivityFix')) to test both flag states in a single test

3. Works with enableViewTransition - The fix correctly handles the ViewTransition early exit path

---

Test Results

• ✅ 142 tests pass with variant=true (enableEventAPIActivityFix ON)
• ✅ 142 tests pass with variant=false (enableEventAPIActivityFix OFF)
• ✅ All tests work correctly with enableViewTransition=true

[react-sizebot]

Comparing: 41b3e9a...b74caa8

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.js	=	6.84 kB	6.84 kB	=	1.88 kB	1.88 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	608.48 kB	608.48 kB	=	107.60 kB	107.60 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.84 kB	6.84 kB	=	1.88 kB	1.88 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	670.95 kB	670.95 kB	=	118.02 kB	118.02 kB
facebook-www/ReactDOM-prod.classic.js	+0.05%	693.87 kB	694.20 kB	+0.03%	121.97 kB	122.02 kB
facebook-www/ReactDOM-prod.modern.js	+0.05%	684.25 kB	684.58 kB	+0.03%	120.37 kB	120.41 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

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[rickhanlonii]

@Ranamalsingh12 future AI generated comment spam will result in a ban.