Skip unchanged children when slicing in the differ (#59242)

Summary:
> This touches the differ, so I'm happy to split it if that's easier to review: (1) a refactor that moves the per-child logic of `sliceChildShadowNodeViewPairs` into helpers, with no behavior change, and (2) the change below, behind the new feature flag.

When a node changes, the differ (`calculateShadowViewMutations`, `ReactCommon/react/renderer/mounting/Differentiator.cpp`) slices the children of its old and new revision with `sliceChildShadowNodeViewPairs` and compares the two lists. A commit clones only the path from the changed node to the root, so most of those children are the same `ShadowNode` in both revisions.

**Current behavior.** Slicing builds a `ShadowViewNodePair`, including a `ShadowView`, for every child in both lists, recursing into flattened children, before the comparison finds that a child is unchanged and skips it. While a list scrolls, the ScrollView writes its content offset into its state on every frame, so every frame produces a commit that changes one node deep in the tree, and the differ slices every sibling along the path to it, twice. In the RNTester chat scenario below, 36 of the 56 child pairs compared in such a diff are the same node in both revisions.

**With this change.** Behind the new `skipUnchangedChildrenInDifferentiator` feature flag (default `false`), the differ slices the children of a matched pair side by side with the new `sliceChangedChildShadowNodeViewPairs` and leaves out each child that is the same node in both revisions at the same offset. Such a child and its flattened descendants give equal pairs in both lists, which the differ matches without a mutation, so the mutations computed from the shorter lists are the same. A child is kept when its offset differs, because a flattened ancestor moved and its frame has to be updated. The side-by-side slicing gives up, and the differ slices both revisions in full as before, when the revisions differ in a way that could change the order or the kind of the pairs: child count, tags, flattening, position type, order index (`zIndex`), visibility or `ChildrenFormStackingContext`, or when view culling is enabled.

The per-child logic of the existing slicer moves into helpers that both slicers share, so they can't drift apart.

## Changelog:

[Internal]

Pull Request resolved: #59242

Test Plan:
- `clang-format --dry-run --Werror` on the changed C++ files passes. `yarn featureflags` regenerated the accessors.
- **Fantom.** `Mounting-itest.js` now runs with `fantom_flags skipUnchangedChildrenInDifferentiator:*`, so every existing differ test also runs with the flag on, and it adds tests for an unchanged child next to a changed one, an unchanged child of a flattened view that moves, a scrolled ScrollView with unchanged siblings, a child added next to unchanged children and a child whose `zIndex` changes. `yarn fantom Mounting-itest`: 56 passed (14 tests × 4 flag combinations). The full suite (`FANTOM_FORCE_CI_MODE=1 yarn fantom`) passes with the flag's default switched to `true` locally, so that every test runs with it on (3,325 passed, 108 skipped), and with it off (same counts), twice each. In a first run with the flag on, the tester crashed once in `setUpDefaultReactNativeEnvironment-Globals-ResizeObserver-itest.js`; it did not crash again in three runs of that file alone or in two more full runs.
- **Same mutations on device.** With a local checker that ran the differ twice for every commit, with and without the side-by-side slicing, and compared the full mutation lists, about 7,500 commits in RNTester on a physical iPhone produced no difference: mounting and unmounting 1000 views, re-rendering, a reflow that moves siblings, opacity animation, list item updates, `removeClippedSubviews`, `adjustsFontSizeToFit` text updates, scrolling a chat FlatList, typing in a TextInput, FlatList and SectionList scrolling.
- Android was not built or tested. The slicing is shared C++, and the Android-specific branches (`useTraitHiddenOnAndroid`) are mirrored from the existing slicer.
- **Measurement.** RNTester built with `xcodebuild` in Release and run on a physical iPhone 15 Pro (iOS 26.6.1). The two arms are the same binary with the flag switched by a launch argument, in the order before, after, after, before. Scrolling was driven by [Argent](https://github.com/software-mansion/argent) on the physical iPhone. Timed: the diff phase of each mount transaction from `TransactionTelemetry`, on the main thread.

| RNTester Playground, a FlatList of 500 multi-line messages, 20 flings down and 20 up per launch (~8,800 transactions) | before | this PR | change |
|---|--:|--:|--:|
| diff time per launch | 1,102 ms | 518 ms | −53.4% |
| diff time per transaction | 126.1 µs | 58.8 µs | −53.4% |
| diff time per 1–2-mutation transaction (the per-frame scroll updates) | 107.2 µs | 44.5 µs | −58.4% |

| RNTester Playground, 1000 labels mounted and unmounted, 10 times per arm | before | this PR | change |
|---|--:|--:|--:|
| diff time per mount or unmount (median) | 2.54 ms | 2.45 ms | within noise |

A mount gains nothing, since every node is new; the second table shows that giving up on the side-by-side slicing costs nothing measurable.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed By: christophpurrer

Differential Revision: D126471123

Pulled By: javache

fbshipit-source-id: a5e3e44e749ab82d9900f5d1ef65c5970deebe1c

Author: hryhoriiK97

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlags.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<<a9fc02c20449f356bba332d1d3c4b25b>>
+ * @generated SignedSource<<574a9a84e11574df2358ed087516b9dc>>
  */
 
 /**
@@ -504,6 +504,12 @@ public object ReactNativeFeatureFlags {
   @JvmStatic
   public fun skipActivityIdentityAssertionOnHostPause(): Boolean = accessor.skipActivityIdentityAssertionOnHostPause()
 
+  /**
+   * When a node changes, the differ leaves out its children that are unchanged in both trees instead of describing and comparing them.
+   */
+  @JvmStatic
+  public fun skipUnchangedChildrenInDifferentiator(): Boolean = accessor.skipUnchangedChildrenInDifferentiator()
+
   /**
    * Override getClipBounds on Android views to return the padding box when overflow is hidden
    */

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlagsCxxAccessor.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<<1d46693f9d74064a63d906637b20c108>>
+ * @generated SignedSource<<d2d170d333194ce2f2a4f1d93321ac6d>>
  */
 
 /**
@@ -99,6 +99,7 @@ internal class ReactNativeFeatureFlagsCxxAccessor : ReactNativeFeatureFlagsAcces
   private var shouldPressibilityUseW3CPointerEventsForHoverCache: Boolean? = null
   private var shouldTriggerResponderTransferOnScrollAndroidCache: Boolean? = null
   private var skipActivityIdentityAssertionOnHostPauseCache: Boolean? = null
+  private var skipUnchangedChildrenInDifferentiatorCache: Boolean? = null
   private var syncAndroidClipBoundsWithOverflowCache: Boolean? = null
   private var traceTurboModulePromiseRejectionsOnAndroidCache: Boolean? = null
   private var updateRuntimeShadowNodeReferencesOnCommitCache: Boolean? = null
@@ -827,6 +828,15 @@ internal class ReactNativeFeatureFlagsCxxAccessor : ReactNativeFeatureFlagsAcces
     return cached
   }
 
+  override fun skipUnchangedChildrenInDifferentiator(): Boolean {
+    var cached = skipUnchangedChildrenInDifferentiatorCache
+    if (cached == null) {
+      cached = ReactNativeFeatureFlagsCxxInterop.skipUnchangedChildrenInDifferentiator()
+      skipUnchangedChildrenInDifferentiatorCache = cached
+    }
+    return cached
+  }
+
   override fun syncAndroidClipBoundsWithOverflow(): Boolean {
     var cached = syncAndroidClipBoundsWithOverflowCache
     if (cached == null) {

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlagsCxxInterop.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<<d6fb6eda2b264730439d587e0f9da3b0>>
+ * @generated SignedSource<<ec3e384af62c0e672da53b57354bca62>>
  */
 
 /**
@@ -186,6 +186,8 @@ public object ReactNativeFeatureFlagsCxxInterop {
 
   @DoNotStrip @JvmStatic public external fun skipActivityIdentityAssertionOnHostPause(): Boolean
 
+  @DoNotStrip @JvmStatic public external fun skipUnchangedChildrenInDifferentiator(): Boolean
+
   @DoNotStrip @JvmStatic public external fun syncAndroidClipBoundsWithOverflow(): Boolean
 
   @DoNotStrip @JvmStatic public external fun traceTurboModulePromiseRejectionsOnAndroid(): Boolean

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<<e5a314c2b7c0a124a805641ca8d8b296>>
+ * @generated SignedSource<<0151a6981c002284da2c1415de4e968e>>
  */
 
 /**
@@ -181,6 +181,8 @@ public open class ReactNativeFeatureFlagsDefaults : ReactNativeFeatureFlagsProvi
 
   override fun skipActivityIdentityAssertionOnHostPause(): Boolean = false
 
+  override fun skipUnchangedChildrenInDifferentiator(): Boolean = false
+
   override fun syncAndroidClipBoundsWithOverflow(): Boolean = false
 
   override fun traceTurboModulePromiseRejectionsOnAndroid(): Boolean = false

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlagsLocalAccessor.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<<8ac941ef9289ab543d0adca8bf06a28d>>
+ * @generated SignedSource<<c36b0b2d7173aa78b7cb175658fc89d2>>
  */
 
 /**
@@ -103,6 +103,7 @@ internal class ReactNativeFeatureFlagsLocalAccessor : ReactNativeFeatureFlagsAcc
   private var shouldPressibilityUseW3CPointerEventsForHoverCache: Boolean? = null
   private var shouldTriggerResponderTransferOnScrollAndroidCache: Boolean? = null
   private var skipActivityIdentityAssertionOnHostPauseCache: Boolean? = null
+  private var skipUnchangedChildrenInDifferentiatorCache: Boolean? = null
   private var syncAndroidClipBoundsWithOverflowCache: Boolean? = null
   private var traceTurboModulePromiseRejectionsOnAndroidCache: Boolean? = null
   private var updateRuntimeShadowNodeReferencesOnCommitCache: Boolean? = null
@@ -910,6 +911,16 @@ internal class ReactNativeFeatureFlagsLocalAccessor : ReactNativeFeatureFlagsAcc
     return cached
   }
 
+  override fun skipUnchangedChildrenInDifferentiator(): Boolean {
+    var cached = skipUnchangedChildrenInDifferentiatorCache
+    if (cached == null) {
+      cached = currentProvider.skipUnchangedChildrenInDifferentiator()
+      accessedFeatureFlags.add("skipUnchangedChildrenInDifferentiator")
+      skipUnchangedChildrenInDifferentiatorCache = cached
+    }
+    return cached
+  }
+
   override fun syncAndroidClipBoundsWithOverflow(): Boolean {
     var cached = syncAndroidClipBoundsWithOverflowCache
     if (cached == null) {

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlagsProvider.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<<db69be97c6b66b5384ddbf25ecf53eb2>>
+ * @generated SignedSource<<c5e9b41b76c5580c0a4ba848af4e36a5>>
  */
 
 /**
@@ -181,6 +181,8 @@ public interface ReactNativeFeatureFlagsProvider {
 
   @DoNotStrip public fun skipActivityIdentityAssertionOnHostPause(): Boolean
 
+  @DoNotStrip public fun skipUnchangedChildrenInDifferentiator(): Boolean
+
   @DoNotStrip public fun syncAndroidClipBoundsWithOverflow(): Boolean
 
   @DoNotStrip public fun traceTurboModulePromiseRejectionsOnAndroid(): Boolean

packages/react-native/ReactAndroid/src/main/jni/react/featureflags/JReactNativeFeatureFlagsCxxInterop.cpp

@@ -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<<f21f6a613ae5168de1d170cdde572f25>>
+ * @generated SignedSource<<1ab2eec7b9c5bdd181ec5fbfd37149ab>>
  */
 
 /**
@@ -513,6 +513,12 @@ class ReactNativeFeatureFlagsJavaProvider
     return method(javaProvider_);
   }
 
+  bool skipUnchangedChildrenInDifferentiator() override {
+    static const auto method =
+        getReactNativeFeatureFlagsProviderJavaClass()->getMethod<jboolean()>("skipUnchangedChildrenInDifferentiator");
+    return method(javaProvider_);
+  }
+
   bool syncAndroidClipBoundsWithOverflow() override {
     static const auto method =
         getReactNativeFeatureFlagsProviderJavaClass()->getMethod<jboolean()>("syncAndroidClipBoundsWithOverflow");
@@ -1008,6 +1014,11 @@ bool JReactNativeFeatureFlagsCxxInterop::skipActivityIdentityAssertionOnHostPaus
   return ReactNativeFeatureFlags::skipActivityIdentityAssertionOnHostPause();
 }
 
+bool JReactNativeFeatureFlagsCxxInterop::skipUnchangedChildrenInDifferentiator(
+    facebook::jni::alias_ref<JReactNativeFeatureFlagsCxxInterop> /*unused*/) {
+  return ReactNativeFeatureFlags::skipUnchangedChildrenInDifferentiator();
+}
+
 bool JReactNativeFeatureFlagsCxxInterop::syncAndroidClipBoundsWithOverflow(
     facebook::jni::alias_ref<JReactNativeFeatureFlagsCxxInterop> /*unused*/) {
   return ReactNativeFeatureFlags::syncAndroidClipBoundsWithOverflow();
@@ -1356,6 +1367,9 @@ void JReactNativeFeatureFlagsCxxInterop::registerNatives() {
       makeNativeMethod(
         "skipActivityIdentityAssertionOnHostPause",
         JReactNativeFeatureFlagsCxxInterop::skipActivityIdentityAssertionOnHostPause),
+      makeNativeMethod(
+        "skipUnchangedChildrenInDifferentiator",
+        JReactNativeFeatureFlagsCxxInterop::skipUnchangedChildrenInDifferentiator),
       makeNativeMethod(
         "syncAndroidClipBoundsWithOverflow",
         JReactNativeFeatureFlagsCxxInterop::syncAndroidClipBoundsWithOverflow),

packages/react-native/ReactAndroid/src/main/jni/react/featureflags/JReactNativeFeatureFlagsCxxInterop.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<<997a74618b4a955bce1ac6ef14b203aa>>
+ * @generated SignedSource<<2c8f9205f1de55591ede3845ba2fa877>>
  */
 
 /**
@@ -267,6 +267,9 @@ class JReactNativeFeatureFlagsCxxInterop
   static bool skipActivityIdentityAssertionOnHostPause(
     facebook::jni::alias_ref<JReactNativeFeatureFlagsCxxInterop>);
 
+  static bool skipUnchangedChildrenInDifferentiator(
+    facebook::jni::alias_ref<JReactNativeFeatureFlagsCxxInterop>);
+
   static bool syncAndroidClipBoundsWithOverflow(
     facebook::jni::alias_ref<JReactNativeFeatureFlagsCxxInterop>);
 

packages/react-native/ReactCommon/react/featureflags/ReactNativeFeatureFlags.cpp

@@ -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<<a49f9bb223f576378418f4b7845ca98f>>
+ * @generated SignedSource<<7542508e6a3b0dd942557748186527b3>>
  */
 
 /**
@@ -342,6 +342,10 @@ bool ReactNativeFeatureFlags::skipActivityIdentityAssertionOnHostPause() {
   return getAccessor().skipActivityIdentityAssertionOnHostPause();
 }
 
+bool ReactNativeFeatureFlags::skipUnchangedChildrenInDifferentiator() {
+  return getAccessor().skipUnchangedChildrenInDifferentiator();
+}
+
 bool ReactNativeFeatureFlags::syncAndroidClipBoundsWithOverflow() {
   return getAccessor().syncAndroidClipBoundsWithOverflow();
 }

packages/react-native/ReactCommon/react/featureflags/ReactNativeFeatureFlags.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<<d77173a9237cebe4468236f6a52980a0>>
+ * @generated SignedSource<<ec78b5d1f026f52f188c7592f6f6bae8>>
  */
 
 /**
@@ -436,6 +436,11 @@ class ReactNativeFeatureFlags {
    */
   RN_EXPORT static bool skipActivityIdentityAssertionOnHostPause();
 
+  /**
+   * When a node changes, the differ leaves out its children that are unchanged in both trees instead of describing and comparing them.
+   */
+  RN_EXPORT static bool skipUnchangedChildrenInDifferentiator();
+
   /**
    * Override getClipBounds on Android views to return the padding box when overflow is hidden
    */