Replace merge commit scheduling with a run loop observer

Summary:
Changelog: [IOS][CHANGED] - Drain React-revision merges from a `BeforeWaiting` main run loop observer instead of dispatching each merge as a separate main-queue block

With Fabric commit branching (`enableFabricCommitBranching`), React commits land on a forked `currentReactRevision_` and are merged into the main branch later via `ShadowTree::mergeReactRevision()`. On iOS, the `schedulerShouldMergeReactRevision:` callback used to do a plain `RCTExecuteOnMainQueue` per promotion, so every promotion enqueued a fresh main queue block that competed for ordering with mount blocks dispatched from concurrent commits with `mountSynchronously=true`.

Instead of dispatching each merge as its own main-queue block, enqueue the surface id into an `unordered_set<SurfaceId>` and drain it from a `MainRunLoopObserver` registered at `kCFRunLoopBeforeWaiting`. The merge calls `ShadowTree::commit(..., mountSynchronously = true)`, so the mount completes inline before the observer returns.

A similar mechanism has already been implemented on Android in D100966623

Differential Revision: D104227839

Author: j-piasecki

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

@@ -9,6 +9,7 @@
 
 #import <mutex>
 #import <shared_mutex>
+#import <unordered_set>
 
 #import <React/RCTAssert.h>
 #import <React/RCTBridge+Private.h>
@@ -42,8 +43,28 @@
 using namespace facebook::react;
 
 @interface RCTSurfacePresenter () <RCTSchedulerDelegate, RCTMountingManagerDelegate>
+- (void)_drainPendingReactRevisionMerges;
 @end
 
+namespace {
+class ReactRevisionMergeRunLoopObserverDelegate final : public RunLoopObserver::Delegate {
+ public:
+  explicit ReactRevisionMergeRunLoopObserverDelegate(RCTSurfacePresenter *surfacePresenter)
+      : surfacePresenter_(surfacePresenter)
+  {
+  }
+
+  void activityDidChange(const RunLoopObserver::Delegate * /*delegate*/, RunLoopObserver::Activity /*activity*/)
+      const noexcept override
+  {
+    [surfacePresenter_ _drainPendingReactRevisionMerges];
+  }
+
+ private:
+  __weak RCTSurfacePresenter *surfacePresenter_;
+};
+} // namespace
+
 @implementation RCTSurfacePresenter {
   RCTMountingManager *_mountingManager; // Thread-safe.
   RCTSurfaceRegistry *_surfaceRegistry; // Thread-safe.
@@ -57,6 +78,12 @@ @implementation RCTSurfacePresenter {
 
   std::shared_mutex _observerListMutex;
   std::vector<__weak id<RCTSurfacePresenterObserver>> _observers; // Protected by `_observerListMutex`.
+
+  std::mutex _pendingReactRevisionMergesMutex;
+  // Pending React revision merges, drained on the main run loop just before it sleeps.
+  std::unordered_set<SurfaceId> _pendingReactRevisionMerges; // Protected by `_pendingReactRevisionMergesMutex`.
+  std::shared_ptr<ReactRevisionMergeRunLoopObserverDelegate> _mergeRunLoopObserverDelegate;
+  std::unique_ptr<const MainRunLoopObserver> _mergeRunLoopObserver;
 }
 
 - (instancetype)initWithContextContainer:(std::shared_ptr<const ContextContainer>)contextContainer
@@ -76,6 +103,13 @@ - (instancetype)initWithContextContainer:(std::shared_ptr<const ContextContainer
 
     _scheduler = [self _createScheduler];
 
+    if (ReactNativeFeatureFlags::enableFabricCommitBranching()) {
+      _mergeRunLoopObserverDelegate = std::make_shared<ReactRevisionMergeRunLoopObserverDelegate>(self);
+      _mergeRunLoopObserver = std::make_unique<const MainRunLoopObserver>(
+          RunLoopObserver::Activity::BeforeWaiting, _mergeRunLoopObserverDelegate);
+      _mergeRunLoopObserver->setDelegate(_mergeRunLoopObserverDelegate.get());
+    }
+
     [[NSNotificationCenter defaultCenter] addObserver:self
                                              selector:@selector(_applicationWillTerminate)
                                                  name:UIApplicationWillTerminateNotification
@@ -309,11 +343,56 @@ - (void)schedulerShouldRenderTransactions:(std::shared_ptr<const MountingCoordin
 
 - (void)schedulerShouldMergeReactRevision:(SurfaceId)surfaceId
 {
-  auto scheduler = [self scheduler];
-  RCTExecuteOnMainQueue(^{
-    scheduler.uiManager->getShadowTreeRegistry().visit(
-        surfaceId, [](const ShadowTree &shadowTree) { shadowTree.mergeReactRevision(); });
-  });
+  if (RCTIsMainQueue()) {
+    [self _mergeReactRevisionForSurfaceId:surfaceId];
+    return;
+  }
+
+  {
+    // Hold the mutex across `enable()` to serialize with `disable()` in
+    // `_drainPendingReactRevisionMerges`.
+    std::lock_guard<std::mutex> lock(_pendingReactRevisionMergesMutex);
+    _pendingReactRevisionMerges.insert(surfaceId);
+    _mergeRunLoopObserver->enable();
+  }
+
+  CFRunLoopWakeUp(CFRunLoopGetMain());
+}
+
+- (void)_mergeReactRevisionForSurfaceId:(SurfaceId)surfaceId
+{
+  RCTAssertMainQueue();
+  RCTScheduler *scheduler = [self scheduler];
+  if (!scheduler) {
+    return;
+  }
+
+  auto uiManager = scheduler.uiManager;
+  if (!uiManager) {
+    return;
+  }
+
+  uiManager->getShadowTreeRegistry().visit(
+      surfaceId, [](const ShadowTree &shadowTree) { shadowTree.mergeReactRevision(); });
+}
+
+- (void)_drainPendingReactRevisionMerges
+{
+  RCTAssertMainQueue();
+
+  std::unordered_set<SurfaceId> pending;
+  {
+    std::lock_guard<std::mutex> lock(_pendingReactRevisionMergesMutex);
+    pending.swap(_pendingReactRevisionMerges);
+
+    // The queue is empty, so disable the observer to avoid waking on every
+    // run loop tick. It's re-enabled on the next enqueue.
+    _mergeRunLoopObserver->disable();
+  }
+
+  for (auto surfaceId : pending) {
+    [self _mergeReactRevisionForSurfaceId:surfaceId];
+  }
 }
 
 - (void)schedulerDidDispatchCommand:(const ShadowView &)shadowView