Add test for synchronous mount props override behavior

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlagsDefaults.kt

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<d2b14345bf627e35562530912b3aae1f>>
+ * @generated SignedSource<<a9a8ce443fa160a7494fc1c9e7baa02f>>
  */
 
 /**
@@ -153,7 +153,7 @@ public open class ReactNativeFeatureFlagsDefaults : ReactNativeFeatureFlagsProvi
 
   override fun hideOffscreenVirtualViewsOnIOS(): Boolean = false
 
-  override fun overrideBySynchronousMountPropsAtMountingAndroid(): Boolean = false
+  override fun overrideBySynchronousMountPropsAtMountingAndroid(): Boolean = true
 
   override fun perfIssuesEnabled(): Boolean = false
 

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

@@ -0,0 +1,178 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+@file:Suppress("DEPRECATION")
+
+package com.facebook.react.fabric
+
+import com.facebook.react.ReactRootView
+import com.facebook.react.bridge.JavaOnlyMap
+import com.facebook.react.bridge.ReactTestHelper
+import com.facebook.react.fabric.mounting.MountingManager
+import com.facebook.react.fabric.mounting.MountingManager.MountItemExecutor
+import com.facebook.react.fabric.mounting.SurfaceMountingManager
+import com.facebook.react.internal.featureflags.ReactNativeFeatureFlagsForTests
+import com.facebook.react.uimanager.ThemedReactContext
+import com.facebook.react.uimanager.ViewManager
+import com.facebook.react.uimanager.ViewManagerRegistry
+import com.facebook.react.views.view.ReactViewManager
+import com.facebook.testutils.shadows.ShadowNativeLoader
+import com.facebook.testutils.shadows.ShadowNativeMap
+import com.facebook.testutils.shadows.ShadowReadableNativeArray
+import com.facebook.testutils.shadows.ShadowReadableNativeMap
+import com.facebook.testutils.shadows.ShadowSoLoader
+import com.facebook.testutils.shadows.ShadowWritableNativeArray
+import com.facebook.testutils.shadows.ShadowWritableNativeMap
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.annotation.Config
+
+/**
+ * Tests for the synchronous mount props override behavior in [SurfaceMountingManager], controlled
+ * by the `overrideBySynchronousMountPropsAtMountingAndroid` feature flag (default: true).
+ *
+ * This fixes a race condition where Native Animated applies props (e.g. opacity, transform)
+ * synchronously on native view, but a regular Fabric mount update with stale props arrives later
+ * and overwrites the fresh values, causing the view to visibly jump/flicker.
+ */
+@RunWith(RobolectricTestRunner::class)
+@Config(
+    shadows =
+        [
+            ShadowSoLoader::class,
+            ShadowNativeLoader::class,
+            ShadowNativeMap::class,
+            ShadowWritableNativeMap::class,
+            ShadowReadableNativeMap::class,
+            ShadowWritableNativeArray::class,
+            ShadowReadableNativeArray::class,
+        ]
+)
+class SurfaceMountingManagerSynchronousMountPropsTest {
+  private lateinit var mountingManager: MountingManager
+  private lateinit var themedReactContext: ThemedReactContext
+  private val surfaceId = 1
+
+  @Before
+  fun setUp() {
+    ReactNativeFeatureFlagsForTests.setUp()
+    val reactContext = ReactTestHelper.createCatalystContextForTest()
+    themedReactContext = ThemedReactContext(reactContext, reactContext, null, -1)
+    mountingManager =
+        MountingManager(
+            ViewManagerRegistry(listOf<ViewManager<*, *>>(ReactViewManager())),
+            MountItemExecutor {},
+        )
+  }
+
+  private fun startSurface(): SurfaceMountingManager {
+    val rootView = ReactRootView(themedReactContext)
+    mountingManager.startSurface(surfaceId, themedReactContext, rootView)
+    return mountingManager.getSurfaceManagerEnforced(surfaceId, "test")
+  }
+
+  private fun createAndAttachView(smm: SurfaceMountingManager, tag: Int) {
+    smm.preallocateView("RCTView", tag, JavaOnlyMap.of(), null, true)
+    smm.addViewAt(surfaceId, tag, 0)
+  }
+
+  /** Stored synchronous opacity should override a stale Fabric mount update. */
+  @Test
+  fun storeSynchronousProps_overridesStaleOpacityInUpdateProps() {
+    val smm = startSurface()
+    val tag = 42
+    createAndAttachView(smm, tag)
+
+    // Native Animated sets opacity=0.3 synchronously
+    smm.storeSynchronousMountPropsOverride(tag, JavaOnlyMap.of("opacity", 0.3))
+
+    // Stale Fabric mount update arrives with opacity=1.0
+    smm.updateProps(tag, JavaOnlyMap.of("opacity", 1.0))
+
+    // The synchronous value (0.3) should win
+    assertThat(smm.getView(tag).alpha).isEqualTo(0.3f)
+  }
+
+  /** Multiple storeSynchronousMountPropsOverride calls should merge — later values win. */
+  @Test
+  fun storeSynchronousProps_mergesMultipleCalls() {
+    val smm = startSurface()
+    val tag = 42
+    createAndAttachView(smm, tag)
+
+    smm.storeSynchronousMountPropsOverride(tag, JavaOnlyMap.of("opacity", 0.3))
+    smm.storeSynchronousMountPropsOverride(tag, JavaOnlyMap.of("opacity", 0.7))
+
+    smm.updateProps(tag, JavaOnlyMap.of("opacity", 1.0))
+
+    assertThat(smm.getView(tag).alpha).isEqualTo(0.7f)
+  }
+
+  /**
+   * Full race condition scenario: synchronous animated props survive a stale Fabric mount update.
+   */
+  @Test
+  fun raceCondition_synchronousPropsWinOverStaleMount() {
+    val smm = startSurface()
+    val tag = 42
+    createAndAttachView(smm, tag)
+
+    // Native Animated applies fresh props synchronously
+    val freshAnimatedProps = JavaOnlyMap.of("opacity", 0.2)
+    smm.storeSynchronousMountPropsOverride(tag, freshAnimatedProps)
+    smm.updatePropsSynchronously(tag, freshAnimatedProps)
+    assertThat(smm.getView(tag).alpha).isEqualTo(0.2f)
+
+    // Stale Fabric mount update arrives
+    smm.updateProps(tag, JavaOnlyMap.of("opacity", 1.0))
+
+    // Synchronous value preserved
+    assertThat(smm.getView(tag).alpha).isEqualTo(0.2f)
+  }
+
+  /**
+   * When a view is deleted, stored synchronous props should be cleaned up. A recreated view with
+   * the same tag should not be affected by the old stored props.
+   */
+  @Test
+  fun deleteView_cleansUpStoredSynchronousProps() {
+    val smm = startSurface()
+    val tag = 42
+    createAndAttachView(smm, tag)
+
+    smm.storeSynchronousMountPropsOverride(tag, JavaOnlyMap.of("opacity", 0.3))
+    smm.deleteView(tag)
+
+    // Recreate with same tag
+    smm.createView("RCTView", tag, JavaOnlyMap.of(), null, null, true)
+    smm.addViewAt(surfaceId, tag, 0)
+
+    smm.updateProps(tag, JavaOnlyMap.of("opacity", 0.9))
+    assertThat(smm.getView(tag).alpha).isEqualTo(0.9f)
+  }
+
+  /** Synchronous props stored for one tag should not affect a different tag. */
+  @Test
+  fun synchronousPropsAreIsolatedPerTag() {
+    val smm = startSurface()
+    createAndAttachView(smm, 42)
+    createAndAttachView(smm, 43)
+
+    smm.storeSynchronousMountPropsOverride(42, JavaOnlyMap.of("opacity", 0.3))
+
+    smm.updateProps(42, JavaOnlyMap.of("opacity", 1.0))
+    smm.updateProps(43, JavaOnlyMap.of("opacity", 1.0))
+
+    // Tag 42: synchronous override applies (0.3)
+    assertThat(smm.getView(42).alpha).isEqualTo(0.3f)
+    // Tag 43: no override, incoming props apply (1.0)
+    assertThat(smm.getView(43).alpha).isEqualTo(1.0f)
+  }
+}

packages/react-native/ReactCommon/react/featureflags/ReactNativeFeatureFlagsDefaults.h

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<4a2fd61cbcdb28042f09ccb03c970674>>
+ * @generated SignedSource<<d987528598996fc7b1bf3c872f51e2ed>>
  */
 
 /**
@@ -288,7 +288,7 @@ class ReactNativeFeatureFlagsDefaults : public ReactNativeFeatureFlagsProvider {
   }
 
   bool overrideBySynchronousMountPropsAtMountingAndroid() override {
-    return false;
+    return true;
   }
 
   bool perfIssuesEnabled() override {

packages/react-native/scripts/featureflags/ReactNativeFeatureFlags.config.js

@@ -743,7 +743,7 @@ const definitions: FeatureFlagDefinitions = {
       ossReleaseStage: 'none',
     },
     overrideBySynchronousMountPropsAtMountingAndroid: {
-      defaultValue: false,
+      defaultValue: true,
       metadata: {
         dateAdded: '2025-09-04',
         description:

packages/react-native/src/private/featureflags/ReactNativeFeatureFlags.js

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<aa202d346e68c3dd641d557306964ebe>>
+ * @generated SignedSource<<1dd51a152bb30c0e2073a14566c8368d>>
  * @flow strict
  * @noformat
  */
@@ -468,7 +468,7 @@ export const hideOffscreenVirtualViewsOnIOS: Getter<boolean> = createNativeFlagG
 /**
  * Override props at mounting with synchronously mounted (i.e. direct manipulation) props from Native Animated.
  */
-export const overrideBySynchronousMountPropsAtMountingAndroid: Getter<boolean> = createNativeFlagGetter('overrideBySynchronousMountPropsAtMountingAndroid', false);
+export const overrideBySynchronousMountPropsAtMountingAndroid: Getter<boolean> = createNativeFlagGetter('overrideBySynchronousMountPropsAtMountingAndroid', true);
 /**
  * Enable reporting Performance Issues (`detail.devtools.performanceIssue`). Displayed in the V2 Performance Monitor and the "Performance Issues" sub-panel in DevTools.
  */