Schedule React revision merge to happen before dispatching events (#56449)

Summary:

Changelog: [ANDROID][FIXED] Schedule React revision merge to happen on `DISPATCH_UI` choreographer queue, before dispatching events

Before the introduction of branching, it was possible to handle synchronous events on the same frame they were dispatched. Introduction of branching broke that because merge was scheduled using `runOnUIThread`, which turned out to be after the `DISPATCH_UI` choreographer phase, thus after the dispatch of events.

Reviewed By: rubennorte

Differential Revision: D100966623

Author: j-piasecki

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

@@ -101,6 +101,7 @@
 import java.util.Map;
 import java.util.Queue;
 import java.util.Set;
+import java.util.concurrent.ConcurrentLinkedQueue;
 import java.util.concurrent.CopyOnWriteArrayList;
 
 /**
@@ -192,6 +193,13 @@ public class FabricUIManager
   @ThreadConfined(UI)
   private final Set<SynchronousEvent> mSynchronousEvents = new HashSet<>();
 
+  /**
+   * Queue of surface IDs that need their React revision merged. Drained during doFrame so that
+   * synchronous events dispatched by the merge are processed in the same frame.
+   */
+  private final ConcurrentLinkedQueue<Integer> mPendingReactRevisionMerges =
+      new ConcurrentLinkedQueue<>();
+
   /**
    * This is used to keep track of whether or not the FabricUIManager has been destroyed. Once the
    * Catalyst instance is being destroyed, we should cease all operation here.
@@ -956,13 +964,7 @@ private void scheduleReactRevisionMerge(int surfaceId) {
         mBinding.mergeReactRevision(surfaceId);
       }
     } else {
-      UiThreadUtil.runOnUiThread(
-          () -> {
-            FabricUIManagerBinding binding = mBinding;
-            if (binding != null) {
-              binding.mergeReactRevision(surfaceId);
-            }
-          });
+      mPendingReactRevisionMerges.add(surfaceId);
     }
   }
 
@@ -1558,6 +1560,18 @@ public void doFrameGuarded(long frameTimeNanos) {
         return;
       }
 
+      // Drain pending React revision merges first so that animations,
+      // preallocation, and mount items operate against the latest revision.
+      if (ReactNativeFeatureFlags.enableFabricCommitBranching()) {
+        FabricUIManagerBinding binding = mBinding;
+        if (binding != null) {
+          Integer mergeSurfaceId;
+          while ((mergeSurfaceId = mPendingReactRevisionMerges.poll()) != null) {
+            binding.mergeReactRevision(mergeSurfaceId);
+          }
+        }
+      }
+
       // Drive any animations from C++.
       // There is a race condition here between getting/setting
       // `mDriveCxxAnimations` which shouldn't matter; it's safe to call