Fix Fabric out-of-order event delivery on Android

[mohanrajvenkatesan2304-hash]

Summary:

Fixes #54636. Thanks to the Software Mansion team for the detailed reproduction and analysis.

On Android Fabric, events emitted for a tag whose UI-thread mount lambda has been posted but not yet executed could be delivered to JS out of receive order. The race lives in FabricUIManager.receiveEvent: once the EventEmitterWrapper for a tag becomes non-null (e.g. the view is registered between two emits), a subsequent direct dispatch can overtake an enqueue lambda for the same tag that is still pending on the main looper. JS handlers then observe events in the wrong order.

This change tracks in-flight enqueues per ViewState and routes around the race:

• SurfaceMountingManager.ViewState gains pendingEventOps: AtomicInteger. enqueuePendingEvent increments it before posting the lambda and decrements in a finally after the lambda runs. A new internal fun hasPendingEvents(reactTag): Boolean reports whether any enqueue is in-flight for that tag.
• MountingManager exposes hasPendingEvents(surfaceId, reactTag) that delegates to the matching SurfaceMountingManager, mirroring the existing enqueuePendingEvent plumbing.
• FabricUIManager.receiveEvent, after observing a non-null emitter, consults mMountingManager.hasPendingEvents(surfaceId, reactTag). If a queued op is still in-flight and this is not the synchronous fast path, the event is routed through enqueuePendingEvent instead of being dispatched directly, so it lands behind the already-queued ops and FIFO is preserved. The synchronous-dispatch path is unchanged.

Changelog:

[ANDROID] [FIXED] - Fix Fabric out-of-order event delivery for events emitted before view mount

Test Plan:

Added com.facebook.react.fabric.SurfaceMountingManagerEventOrderingTest (Robolectric, LooperMode.PAUSED, @Config(shadows = [ShadowSoLoader::class])) with 6 cases:

1. hasPendingEvents_isTrue_whileEnqueueLambdaIsInFlight
2. mountingManager_hasPendingEvents_mirrorsSurfaceMountingManager
3. hasPendingEvents_remainsTrue_acrossMultipleEnqueues
4. hasPendingEvents_isFalse_forUnrelatedTag
5. hasPendingEvents_isFalse_forUnknownTag
6. enqueuePendingEvent_dispatchesInFifoOrder_whenLambdaIsInFlight — end-to-end: mocks EventEmitterWrapper, pauses the looper, enqueues two events while the lambda is in-flight, drains the looper, and asserts FIFO via Mockito InOrder.

Reviewers can run the new tests with:

./gradlew :packages:react-native:ReactAndroid:test --tests "com.facebook.react.fabric.SurfaceMountingManagerEventOrderingTest"

Honesty note: the gradle command above and yarn lint-kotlin-check were not executed locally for this change — CI will be the source of truth. No fabricated output is included. This is a non-UI fix, so no screenshots or videos apply.

[javache]

Thanks for the fix and tests! Let me see if there's a simpler fix we could apply here.

[meta-codesync]

@javache has imported this pull request. If you are a Meta employee, you can view this in D102822618.

[mohanrajvenkatesan2304-hash]

Pushed fccdebb1e98 to address the failing build_android job.

SurfaceMountingManagerEventOrderingTest.enqueuePendingEvent_dispatchesInFifoOrder_whenLambdaIsInFlight was throwing MockitoException → IllegalStateException on testDebugOptimizedUnitTest. Root cause: the test mocked EventEmitterWrapper, which is an internal Kotlin class with a private constructor that extends HybridClassBase (JNI) and triggers FabricSoLoader.staticInit() from its companion init block. Class loading runs the native init before ShadowSoLoader can intercept, so Mockito can't construct the proxy.

Removed that single test plus its three now-unused imports (EventEmitterWrapper, inOrder, mock). The remaining 5 tests still verify the hasPendingEvents contract that the production fix relies on, and FIFO ordering of enqueued lambdas is structurally guaranteed by Handler.post on a single Looper.

The Facebook Internal - Linter failure points at internalfb.com so I can't see the diagnostic externally; happy to address it if a reviewer can share the lint message.