Enable overrideBySynchronousMountPropsAtMountingAndroid by default on Android

[jingjing2222]

Enable overrideBySynchronousMountPropsAtMountingAndroid by default on Android.

This flag was added to prevent regular Fabric mount updates from overriding newer props that were already applied synchronously by Native Animated direct manipulation. Without this path enabled, Android can still hit a race where:

1. Native Animated applies transform props synchronously on the UI thread.
2. A regular Fabric mount update for the same view is already in flight with older props.
3. That regular mount update reaches SurfaceMountingManager.updateProps() later and overwrites the newer synchronous animated value.
4. The view visibly jumps/flickers during the animation.

This is not just theoretical. I spent about a week debugging this from a real Android flickering issue, and the final repro is intentionally very small. The visual impact is clear in the README videos, and I am attaching the same videos here as well.

Minimal repro:
https://github.com/jingjing2222/fix-react-native-android-flickering

The repro uses React Native 0.85.2 with Android Fabric. The JS only runs Animated.timing(..., {useNativeDriver: true}) for translateX and translateY.

As-is

as-is.mov

With the current default, flickering is visible during the animation.

To-be

to-be.mov

With overrideBySynchronousMountPropsAtMountingAndroid enabled, the flickering is gone.

The existing mounting-layer flag fixes this by storing the latest synchronous mount props and reapplying them when regular mount props are processed. This makes the final mounted props preserve the latest Native Animated direct manipulation value.

Because this can happen with a simple native-driver transform animation, enabling this default should prevent other Android users from spending time debugging the same race condition.

Why Android only?

iOS Fabric already tracks props managed by NativeAnimated on the component view. When NativeAnimated synchronously updates props such as transform or opacity, iOS records those prop keys and prevents later normal Fabric updates from re-applying stale values for those props.

Android Fabric has a similar mechanism through tagToSynchronousMountProps, but it was gated behind overrideBySynchronousMountPropsAtMountingAndroid, which defaults to false. Therefore, NativeAnimated could synchronously apply the final transform to the view, and a later normal Fabric props update could re-apply stale transform props, causing a transient flicker.

This change makes Android preserve synchronously mounted NativeAnimated props across later Fabric prop updates, matching the ownership behavior already present on iOS.

Changelog:

[ANDROID] [FIXED] - Prevent stale Fabric mount updates from overriding newer Native Animated synchronous prop updates.

Test Plan:

• Ran yarn featureflags --update
• Ran git diff --check
• Verified generated feature flag defaults were updated for Android, C++, and JS.
• Verified the minimal repro with the README videos attached above:
  • as-is/current default: flickering is visible during translateX/translateY native-driver animation
  • to-be/flag enabled: flickering is not visible

[cortinico]

cc @zeyap

[jingjing2222]

rerun Test All / test_ios_rntester (Release, false) (pull_request) please 🙏

[jingjing2222]

rerun Test All / test_ios_rntester (Release, false) (pull_request) please 🙏

Test All / test_ios_rntester (Release, false) (pull_request) too please 🙏

[zeyap]

Yes from previous experiments this flag was pretty low risk, glad it fixed your problem🫡

[meta-codesync]

@zeyap has imported this pull request. If you are a Meta employee, you can view this in D103070058.

[meta-codesync]

@zeyap merged this pull request in eba0bfb.

[react-native-bot]

This pull request was successfully merged by @jingjing2222 in eba0bfb

^{When will my fix make it into a release? | How to file a pick request?}