Defer the pull model's synchronous mount batch until the root view is attached

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

@@ -971,9 +971,15 @@ private void scheduleMountItem(
     if (shouldSchedule) {
       Assertions.assertNotNull(mountItem, "MountItem is null");
       if (synchronous) {
-        // Pull model: we are already on the UI thread, inside the dispatcher's loop executing
-        // a PullTransactionMountItem. We don't schedule the item, we execute it directly.
-        mountItem.execute(mMountingManager);
+        if (mMountingManager.isWaitingForViewAttach(mountItem.getSurfaceId())) {
+          // Regular mount items are still being deferred into the surface's attach queue.
+          // Executing this batch inline would run it ahead of the preallocations queued there.
+          mMountItemDispatcher.addMountItem(mountItem);
+        } else {
+          // Pull model: we are already on the UI thread, inside the dispatcher's loop executing
+          // a PullTransactionMountItem. We don't schedule the item, we execute it directly.
+          mountItem.execute(mMountingManager);
+        }
       } else {
         mMountItemDispatcher.addMountItem(mountItem);
         if (UiThreadUtil.isOnUiThread()) {

packages/react-native/ReactAndroid/src/test/java/com/facebook/react/fabric/FabricUIManagerPullModelTest.kt

@@ -9,22 +9,30 @@
 
 package com.facebook.react.fabric
 
+import com.facebook.react.ReactRootView
 import com.facebook.react.bridge.ReactApplicationContext
 import com.facebook.react.bridge.ReactTestHelper
+import com.facebook.react.fabric.mounting.MountingManager
+import com.facebook.react.fabric.mounting.mountitems.MountItem
+import com.facebook.react.internal.featureflags.ReactNativeFeatureFlags
+import com.facebook.react.internal.featureflags.ReactNativeFeatureFlagsDefaults
 import com.facebook.react.internal.featureflags.ReactNativeFeatureFlagsForTests
+import com.facebook.react.uimanager.ThemedReactContext
 import com.facebook.react.uimanager.ViewManagerRegistry
 import com.facebook.testutils.fakes.FakeBatchEventDispatchedListener
 import com.facebook.testutils.shadows.ShadowFabricUIManagerBinding
 import com.facebook.testutils.shadows.ShadowNativeLoader
 import com.facebook.testutils.shadows.ShadowPerformanceTracer
 import com.facebook.testutils.shadows.ShadowSoLoader
 import org.assertj.core.api.Assertions.assertThat
+import org.junit.After
 import org.junit.Before
 import org.junit.Test
 import org.junit.runner.RunWith
 import org.robolectric.RobolectricTestRunner
 import org.robolectric.annotation.Config
 import org.robolectric.shadow.api.Shadow
+import org.robolectric.util.ReflectionHelpers
 
 /**
  * Tests for the pull-model notification path: [FabricUIManager.onTransactionAvailable] enqueues a
@@ -55,6 +63,11 @@ class FabricUIManagerPullModelTest {
   @Before
   fun setup() {
     ReactNativeFeatureFlagsForTests.setUp()
+    ReactNativeFeatureFlags.override(
+        object : ReactNativeFeatureFlagsDefaults() {
+          override fun enableMountingCoordinatorPullModelAndroid(): Boolean = true
+        },
+    )
     reactContext = ReactTestHelper.createCatalystContextForTest()
     underTest =
         FabricUIManager(
@@ -67,6 +80,11 @@ class FabricUIManagerPullModelTest {
     underTest.setBinding(binding)
   }
 
+  @After
+  fun tearDown() {
+    ReactNativeFeatureFlags.dangerouslyReset()
+  }
+
   private fun runOnBackgroundThread(block: () -> Unit) {
     var error: Throwable? = null
     val thread = Thread {
@@ -81,6 +99,28 @@ class FabricUIManagerPullModelTest {
     error?.let { throw it }
   }
 
+  private fun scheduleSynchronously(mountItem: MountItem) {
+    val method =
+        FabricUIManager::class
+            .java
+            .getDeclaredMethod(
+                "scheduleMountItem",
+                MountItem::class.java,
+                Integer.TYPE,
+                java.lang.Long.TYPE,
+                java.lang.Long.TYPE,
+                java.lang.Long.TYPE,
+                java.lang.Long.TYPE,
+                java.lang.Long.TYPE,
+                java.lang.Long.TYPE,
+                java.lang.Long.TYPE,
+                Integer.TYPE,
+                java.lang.Boolean.TYPE,
+            )
+    method.isAccessible = true
+    method.invoke(underTest, mountItem, 0, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0, true)
+  }
+
   @Test
   fun onTransactionAvailable_onUiThread_pullsSynchronously() {
     underTest.onTransactionAvailable(1)
@@ -105,4 +145,31 @@ class FabricUIManagerPullModelTest {
 
     assertThat(shadowBinding.pulledSurfaceIds).containsExactly(1, 2, 1, 3)
   }
+
+  @Test
+  fun synchronousBatch_waitingForRootAttach_isDeferred() {
+    val surfaceId = 12
+    val themedReactContext = ThemedReactContext(reactContext, reactContext, "TestModule", surfaceId)
+    val mountingManager = ReflectionHelpers.getField<MountingManager>(underTest, "mMountingManager")
+    mountingManager.startSurface(surfaceId, themedReactContext, null)
+
+    var executionCount = 0
+    val mountItem =
+        object : MountItem {
+          override fun execute(mountingManager: MountingManager) {
+            executionCount++
+          }
+
+          override fun getSurfaceId(): Int = surfaceId
+        }
+
+    scheduleSynchronously(mountItem)
+
+    assertThat(executionCount).isZero()
+
+    mountingManager.attachRootView(surfaceId, ReactRootView(reactContext), themedReactContext)
+    underTest.onTransactionAvailable(surfaceId)
+
+    assertThat(executionCount).isEqualTo(1)
+  }
 }