Update branching implementation to default to the JS thread merge

Summary:
Changelog: [General][Changed] - Updated the default branching path to merge the React revisions on the JS thread

The current behavior, merge on the main thread, is kept behind the new `enableFabricCommitBranchingMergeOnMainThread` flag.

Differential Revision: D120981616

Author: j-piasecki

packages/react-native/React/Fabric/RCTSurfacePresenter.mm

@@ -101,7 +101,7 @@ - (instancetype)initWithContextContainer:(std::shared_ptr<const ContextContainer
     _mountingManager.contextContainer = contextContainer;
     _mountingManager.delegate = self;
 
-    if (ReactNativeFeatureFlags::enableFabricCommitBranching()) {
+    if (ReactNativeFeatureFlags::enableFabricCommitBranchingMergeOnMainThread()) {
       _mergeRunLoopObserverDelegate = std::make_shared<ReactRevisionMergeRunLoopObserverDelegate>(self);
       _mergeRunLoopObserver = std::make_unique<const MainRunLoopObserver>(
           RunLoopObserver::Activity::BeforeWaiting, _mergeRunLoopObserverDelegate);
@@ -350,7 +350,7 @@ - (void)schedulerShouldRenderTransactions:(std::shared_ptr<const MountingCoordin
 
 - (void)schedulerShouldMergeReactRevision:(SurfaceId)surfaceId
 {
-  if (RCTIsMainQueue()) {
+  if (RCTIsMainQueue() || !ReactNativeFeatureFlags::enableFabricCommitBranchingMergeOnMainThread()) {
     [self _mergeReactRevisionForSurfaceId:surfaceId];
     return;
   }
@@ -369,7 +369,6 @@ - (void)schedulerShouldMergeReactRevision:(SurfaceId)surfaceId
 
 - (void)_mergeReactRevisionForSurfaceId:(SurfaceId)surfaceId
 {
-  RCTAssertMainQueue();
   RCTScheduler *scheduler = [self scheduler];
   if (!scheduler) {
     return;

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/FabricUIManager.java

@@ -1628,7 +1628,7 @@ public void doFrameGuarded(long frameTimeNanos) {
 
       // Drain pending React revision merges first so that animations,
       // preallocation, and mount items operate against the latest revision.
-      if (ReactNativeFeatureFlags.enableFabricCommitBranching()) {
+      if (ReactNativeFeatureFlags.enableFabricCommitBranchingMergeOnMainThread()) {
         FabricUIManagerBinding binding = mBinding;
         if (binding != null) {
           Integer mergeSurfaceId;

packages/react-native/ReactAndroid/src/main/jni/react/fabric/FabricUIManagerBinding.cpp

@@ -767,17 +767,24 @@ void FabricUIManagerBinding::schedulerShouldRenderTransactions(
 
 void FabricUIManagerBinding::schedulerShouldMergeReactRevision(
     SurfaceId surfaceId) {
-  std::shared_lock lock(installMutex_);
-  auto mountingManager =
-      getMountingManager("schedulerShouldMergeReactRevision");
-  if (mountingManager) {
-    mountingManager->scheduleReactRevisionMerge(surfaceId);
+  if (ReactNativeFeatureFlags::enableFabricCommitBranchingMergeOnMainThread()) {
+    auto mountingManager =
+        getMountingManager("schedulerShouldMergeReactRevision");
+    if (mountingManager) {
+      mountingManager->scheduleReactRevisionMerge(surfaceId);
+    }
+  } else {
+    mergeReactRevision(surfaceId);
   }
 }
 
 void FabricUIManagerBinding::mergeReactRevision(SurfaceId surfaceId) {
-  std::shared_lock lock(installMutex_);
-  scheduler_->getUIManager()->getShadowTreeRegistry().visit(
+  auto scheduler = getScheduler();
+  if (!scheduler) {
+    return;
+  }
+
+  scheduler->getUIManager()->getShadowTreeRegistry().visit(
       surfaceId,
       [](const ShadowTree& shadowTree) { shadowTree.mergeReactRevision(); });
 }