Skip the unchanged scroll state update when a ScrollView mounts on Android

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactScrollViewHelper.kt

@@ -40,6 +40,7 @@ public object ReactScrollViewHelper {
   private val TAG = ReactScrollView::class.java.simpleName
   private val DEBUG_MODE = false // ReactBuildConfig.DEBUG
 
+  private const val UNSET_SCROLL_POSITION = -1
   private const val CONTENT_OFFSET_LEFT = "contentOffsetLeft"
   private const val CONTENT_OFFSET_TOP = "contentOffsetTop"
   private const val SCROLL_AWAY_PADDING_TOP = "scrollAwayPaddingTop"
@@ -373,6 +374,17 @@ public object ReactScrollViewHelper {
     // User driven scrolling should disable scroll state updates coming from Fabric
     scrollState.isUpdatedByScroll = true
 
+    // Until a view sends its first update it doesn't know the offset in its state. On mount that
+    // offset is usually already correct, and sending it again would only commit an unchanged tree.
+    if (scrollState.lastStateUpdateScroll.equals(UNSET_SCROLL_POSITION, UNSET_SCROLL_POSITION)) {
+      scrollView.stateWrapper?.stateData?.let { stateData ->
+        scrollState.setLastStateUpdateScroll(
+            toPixelFromDIP(stateData.getDouble(CONTENT_OFFSET_LEFT)).toInt(),
+            toPixelFromDIP(stateData.getDouble(CONTENT_OFFSET_TOP)).toInt(),
+        )
+      }
+    }
+
     // Dedupe events to reduce JNI traffic
     if (scrollState.lastStateUpdateScroll.equals(scrollX, scrollY)) {
       return
@@ -660,7 +672,7 @@ public object ReactScrollViewHelper {
       /** Get the padding on the bottom for tab bar */
       var scrollAwayPaddingBottom: Int = 0,
       /** Get the Fabric state of last scroll position */
-      val lastStateUpdateScroll: Point = Point(-1, -1),
+      val lastStateUpdateScroll: Point = Point(UNSET_SCROLL_POSITION, UNSET_SCROLL_POSITION),
       /** Get true if the previous animation was canceled */
       var isCanceled: Boolean = false,
       /** Get true if previous animation was finished */

packages/react-native/ReactAndroid/src/test/java/com/facebook/react/views/scroll/ReactScrollViewHelperStateTest.kt

@@ -0,0 +1,119 @@
+/*
+ * 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.
+ */
+
+package com.facebook.react.views.scroll
+
+import android.annotation.SuppressLint
+import android.util.DisplayMetrics
+import com.facebook.react.bridge.ReactApplicationContext
+import com.facebook.react.bridge.ReactTestHelper.createTestReactApplicationContext
+import com.facebook.react.bridge.ReadableNativeMap
+import com.facebook.react.internal.featureflags.ReactNativeFeatureFlagsForTests
+import com.facebook.react.uimanager.DisplayMetricsHolder
+import com.facebook.react.uimanager.StateWrapper
+import com.facebook.react.uimanager.ThemedReactContext
+import com.facebook.testutils.shadows.ShadowNativeLoader
+import com.facebook.testutils.shadows.ShadowNativeMap
+import com.facebook.testutils.shadows.ShadowReadableNativeMap
+import com.facebook.testutils.shadows.ShadowSoLoader
+import com.facebook.testutils.shadows.ShadowWritableNativeMap
+import org.junit.After
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.mockito.kotlin.any
+import org.mockito.kotlin.doReturn
+import org.mockito.kotlin.mock
+import org.mockito.kotlin.never
+import org.mockito.kotlin.times
+import org.mockito.kotlin.verify
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.annotation.Config
+
+/** Verifies when a ScrollView sends its scroll offset to the Fabric state. */
+@SuppressLint("DeprecatedClass", "DeprecatedMethod")
+@RunWith(RobolectricTestRunner::class)
+@Config(
+    shadows =
+        [
+            ShadowNativeLoader::class,
+            ShadowSoLoader::class,
+            ShadowNativeMap::class,
+            ShadowReadableNativeMap::class,
+            ShadowWritableNativeMap::class,
+        ],
+)
+class ReactScrollViewHelperStateTest {
+
+  private lateinit var view: ReactScrollView
+
+  @Before
+  fun setup() {
+    ReactNativeFeatureFlagsForTests.setUp()
+    val context: ReactApplicationContext = createTestReactApplicationContext()
+    DisplayMetricsHolder.setScreenDisplayMetrics(DisplayMetrics().apply { density = 1f })
+    view =
+        ReactScrollViewManager().createViewInstance(ThemedReactContext(context, context, null, -1))
+  }
+
+  @After
+  fun teardown() {
+    DisplayMetricsHolder.setScreenDisplayMetrics(null)
+  }
+
+  private fun stateWrapperWithOffset(x: Double, y: Double): StateWrapper {
+    val stateData =
+        mock<ReadableNativeMap> {
+          on { getDouble("contentOffsetLeft") } doReturn x
+          on { getDouble("contentOffsetTop") } doReturn y
+        }
+    return mock { on { this.stateData } doReturn stateData }
+  }
+
+  @Test
+  fun testFirstUpdateMatchingTheStateIsNotSent() {
+    val stateWrapper = stateWrapperWithOffset(0.0, 0.0)
+    view.stateWrapper = stateWrapper
+
+    ReactScrollViewHelper.updateFabricScrollState(view, 0, 0)
+
+    verify(stateWrapper, never()).updateState(any())
+  }
+
+  @Test
+  fun testFirstUpdateDifferingFromTheStateIsSent() {
+    val stateWrapper = stateWrapperWithOffset(0.0, 500.0)
+    view.stateWrapper = stateWrapper
+
+    ReactScrollViewHelper.updateFabricScrollState(view, 0, 0)
+
+    verify(stateWrapper, times(1)).updateState(any())
+  }
+
+  @Test
+  fun testFirstUpdateIsSentWithoutStateData() {
+    val stateWrapper = mock<StateWrapper> { on { stateData } doReturn null }
+    view.stateWrapper = stateWrapper
+
+    ReactScrollViewHelper.updateFabricScrollState(view, 0, 0)
+
+    verify(stateWrapper, times(1)).updateState(any())
+  }
+
+  @Test
+  fun testLaterUpdatesAreSentOncePerOffset() {
+    val stateWrapper = stateWrapperWithOffset(0.0, 0.0)
+    view.stateWrapper = stateWrapper
+
+    ReactScrollViewHelper.updateFabricScrollState(view, 0, 0)
+    ReactScrollViewHelper.updateFabricScrollState(view, 0, 100)
+    ReactScrollViewHelper.updateFabricScrollState(view, 0, 100)
+
+    verify(stateWrapper, times(1)).updateState(any())
+    verify(stateWrapper, times(1)).stateData
+  }
+}