Gate fix for T54997838 with a feature flag

Summary:
Gating fix for T54997838 behind a feature flag for experimentation.

Changelog: [Internal]

Reviewed By: mdvacca

Differential Revision: D18534767

fbshipit-source-id: 5c0191c454fba6dd4a36d6e179959651ec197cac

ReactAndroid/src/main/java/com/facebook/react/config/ReactFeatureFlags.java

@@ -70,4 +70,12 @@ public class ReactFeatureFlags {
    * and the CatalystInstance will always be destroyed in some future release.
    */
   public static boolean nullifyCatalystInstanceOnDestroy = false;
+
+  /**
+   * Temporary flag that should be removed soon. See FabricUIManager: if this flag is disabled,
+   * mountItems scheduled on the UI thread will *always* be executed synchronously. If this flag is
+   * enabled, users of FabricUIManager may disable immediate execution of scheduled mount items.
+   * TODO T54997838: remove as followup
+   */
+  public static boolean allowDisablingImmediateExecutionOfScheduleMountItems = false;
 }

ReactAndroid/src/main/java/com/facebook/react/fabric/FabricUIManager.java

@@ -533,8 +533,14 @@ public class FabricUIManager implements UIManager, LifecycleEventListener {
       mMountItems.add(mountItem);
     }
 
-    if (mImmediatelyExecutedMountItemsOnUI && UiThreadUtil.isOnUiThread()) {
-      dispatchMountItems();
+    if (UiThreadUtil.isOnUiThread()) {
+      // We only read these flags on the UI thread.
+      boolean immediateExecutionEnabled =
+          !ReactFeatureFlags.allowDisablingImmediateExecutionOfScheduleMountItems
+              || mImmediatelyExecutedMountItemsOnUI;
+      if (immediateExecutionEnabled) {
+        dispatchMountItems();
+      }
     }
 
     // Post markers outside of lock and after sync mounting finishes its execution