Avoid per-child dynamic_casts during Yoga layout

## Summary:

`YogaLayoutableShadowNode` runs three `dynamic_cast`s for children on every layout pass:

- `resolveErrata` checks `dynamic_cast<const LayoutConformanceShadowNode *>(this)`. `configureYogaTree` calls it for every child to decide whether the child has to be configured again, so it runs on every layout, and it fails for almost every node, which is the slowest case of a `dynamic_cast`.
- `getContentBounds` checks `dynamic_cast<const ViewShadowNode *>` for every child to read its `hitSlop`. It fails for every child that isn't a `View` (Text, ScrollView, Image, …).
- `shadowNodeFromContext` gets the shadow node back from a Yoga node's context with a `dynamic_cast`. It runs for every child in `layout()` and `getContentBounds()`, and in Yoga's measure, baseline and clone callbacks.

**Current behavior.** In Time Profiler traces of JS-driven updates on an iPhone, `__dynamic_cast` took 11–18% of the JS thread, most of it inside `layoutTree`, and the type comparisons in those traces go through `strcmp`.

**With this change.** `LayoutConformanceShadowNode` and `ViewShadowNode` are both `final`, and their nodes are only created by descriptors that report the class's own `Handle()`, so comparing `getComponentHandle()` with that handle gives the same answer as the `dynamic_cast`; a `static_cast` follows. The Yoga node's context is only set by this class's two constructors, to `this`, so `shadowNodeFromContext` recovers it with a `static_cast` of the stored pointer. The behavior doesn't change.

## Changelog:

[GENERAL] [CHANGED] - Yoga layout no longer uses `dynamic_cast` for every child to check its type

## Test Plan:

- The pinned `clang-format` passes on the changed file, and it compiles for iOS (arm64).
- `yarn fantom` (full suite, tester rebuilt with cmake from this change, then from `main`): no test fails with this change that doesn't also fail on `main` in the same local environment. The local failures (LogBox and `expect` snapshot formatting, `Text-itest`, `EventTimingAPI-itest`) happen with and without this change.
- **Measurement.** RNTester built with `xcodebuild` in Release and run on a physical iPhone 15 Pro (iOS 26.6.1). Both arms are the same binary, with this change switched by an environment variable read once. 8 launches in the order before, after, after, before, after, before, before, after; in each, 12 s of Time Profiler per scenario, counting JS-thread samples. Interactions were driven by [Argent](https://github.com/software-mansion/argent) on the physical iPhone. The phone warmed up during the runs, so the totals of the same arm vary by up to 30%; the time in `dynamic_cast` is the most direct number, and the medians are given for context.

RNTester Playground, 30 `Text`s animated with `Animated` (JS driver), medians of 4 launches per arm:

| Scenario | | before | this PR | change |
|---|---|--:|--:|--:|
| Color animated | `dynamic_cast` inside layout, per 12 s | 317 ms | 1.5 ms | −99.5% |
| | `dynamic_cast`, share of the JS thread | 17.0% | 4.9% | |
| | Yoga layout (`layoutTree`), per 12 s | 1,144 ms | 836 ms | −26.9% |
| | JS thread, per 12 s | 2,454 ms | 2,154 ms | −12.2% |
| Color and font size animated | `dynamic_cast` inside layout, per 12 s | 338 ms | 1.5 ms | −99.6% |
| | `dynamic_cast`, share of the JS thread | 11.1% | 3.0% | |
| | Yoga layout (`layoutTree`), per 12 s | 2,618 ms | 2,166 ms | −17.3% |
| | JS thread, per 12 s | 4,098 ms | 3,562 ms | −13.1% |

The `dynamic_cast` time left with this change (about 3–5% of the JS thread) comes from other places, such as `adoptYogaChild` and the shadow node constructors, which this PR doesn't change.

On Android these casts cost much less, since there the type comparison compares pointers: in the same color-animation scenario on an Android emulator, `dynamic_cast` inside layout went from 1.5% to 0.2% of the JS thread.

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

Differential Revision: D128421875

Pull Request resolved: #59643

Author: hryhoriiK97

packages/react-native/Libraries/Components/LayoutConformance/__tests__/LayoutConformance-itest.js

@@ -0,0 +1,72 @@
+/**
+ * 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.
+ *
+ * @flow strict-local
+ * @fantom_flags enableFlexboxAutoMinSizeInStrictMode:*
+ * @format
+ */
+
+import '@react-native/fantom/src/setUpDefaultReactNativeEnvironment';
+
+import type {HostInstance} from 'react-native';
+
+import * as Fantom from '@react-native/fantom';
+import nullthrows from 'nullthrows';
+import * as React from 'react';
+import {createRef} from 'react';
+import {
+  View,
+  experimental_LayoutConformance as LayoutConformance,
+} from 'react-native';
+
+type Mode = 'strict' | 'compatibility';
+
+function renderRowWidth(mode: Mode, outerMode?: Mode): number {
+  const root = Fantom.createRoot();
+  const rowRef = createRef<HostInstance>();
+
+  const content = (
+    <LayoutConformance mode={mode}>
+      <View
+        collapsable={false}
+        style={{
+          width: 60,
+          height: 60,
+          flexDirection: 'row',
+          alignItems: 'center',
+        }}>
+        {/* With YGErrataStretchFlexBasis this row grows to its parent's
+            width, even though its content takes no space. */}
+        <View collapsable={false} ref={rowRef} style={{flexDirection: 'row'}}>
+          <View collapsable={false} style={{height: 30, flexGrow: 1}} />
+        </View>
+      </View>
+    </LayoutConformance>
+  );
+
+  Fantom.runTask(() => {
+    root.render(
+      outerMode != null ? (
+        <LayoutConformance mode={outerMode}>{content}</LayoutConformance>
+      ) : (
+        content
+      ),
+    );
+  });
+
+  const width = nullthrows(rowRef.current).getBoundingClientRect().width;
+  root.destroy();
+  return width;
+}
+
+test('strict mode lays out its subtree without the stretch flex-basis errata', () => {
+  expect(renderRowWidth('strict')).toBe(0);
+});
+
+// Compatibility is also the default, so check it under a strict subtree.
+test('compatibility mode inside strict mode brings the stretch flex-basis errata back', () => {
+  expect(renderRowWidth('compatibility', 'strict')).toBe(60);
+});

packages/react-native/ReactCommon/react/renderer/components/view/LayoutConformanceShadowNode.cpp

@@ -0,0 +1,15 @@
+/*
+ * 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.
+ */
+
+#include "LayoutConformanceShadowNode.h"
+
+namespace facebook::react {
+
+// NOLINTNEXTLINE(facebook-hte-CArray,modernize-avoid-c-arrays)
+const char LayoutConformanceShadowNodeComponentName[] = "LayoutConformance";
+
+} // namespace facebook::react

packages/react-native/ReactCommon/react/renderer/components/view/LayoutConformanceShadowNode.h

@@ -15,7 +15,7 @@
 namespace facebook::react {
 
 // NOLINTNEXTLINE(facebook-hte-CArray,modernize-avoid-c-arrays)
-constexpr const char LayoutConformanceShadowNodeComponentName[] = "LayoutConformance";
+extern const char LayoutConformanceShadowNodeComponentName[];
 
 class LayoutConformanceShadowNode final : public ConcreteShadowNode<
                                               LayoutConformanceShadowNodeComponentName,

packages/react-native/ReactCommon/react/renderer/components/view/YogaLayoutableShadowNode.cpp

@@ -560,9 +560,15 @@ void YogaLayoutableShadowNode::configureYogaTree(
 }
 
 YGErrata YogaLayoutableShadowNode::resolveErrata(YGErrata defaultErrata) const {
-  if (auto layoutConformanceNode =
-          dynamic_cast<const LayoutConformanceShadowNode*>(this)) {
-    switch (layoutConformanceNode->getConcreteProps().mode) {
+  // This runs for every child on every layout, and almost no node is a
+  // LayoutConformance node, so a failing `dynamic_cast` would be the common
+  // case. The class is final and its component name is defined once, in
+  // LayoutConformanceShadowNode.cpp, so comparing component handles is
+  // equivalent.
+  if (getComponentHandle() == LayoutConformanceShadowNode::Handle()) {
+    const auto& layoutConformanceNode =
+        static_cast<const LayoutConformanceShadowNode&>(*this);
+    switch (layoutConformanceNode.getConcreteProps().mode) {
       case LayoutConformance::Strict:
         // CSS Flexbox §4.5 automatic minimum sizing is the spec-compliant
         // behaviour, so strict conformance should adopt it — but its probe
@@ -829,7 +835,12 @@ Rect YogaLayoutableShadowNode::getContentBounds() const {
 
     auto layoutMetricsWithOverflowInset = childNode.getLayoutMetrics();
     if (layoutMetricsWithOverflowInset.displayType != DisplayType::None) {
-      auto viewChildNode = dynamic_cast<const ViewShadowNode*>(&childNode);
+      // ViewShadowNode is final, so comparing component handles is equivalent
+      // to a `dynamic_cast`, without its cost for every non-View child.
+      auto viewChildNode =
+          childNode.getComponentHandle() == ViewShadowNode::Handle()
+          ? static_cast<const ViewShadowNode*>(&childNode)
+          : nullptr;
       auto hitSlop = viewChildNode != nullptr
           ? viewChildNode->getConcreteProps().hitSlop
           : EdgeInsets{};
@@ -951,8 +962,8 @@ float YogaLayoutableShadowNode::yogaNodeBaselineCallbackConnector(
 
 YogaLayoutableShadowNode& YogaLayoutableShadowNode::shadowNodeFromContext(
     YGNodeConstRef yogaNode) {
-  return dynamic_cast<YogaLayoutableShadowNode&>(
-      *static_cast<ShadowNode*>(YGNodeGetContext(yogaNode)));
+  // The context is only set by the constructors of this class, to `this`.
+  return *static_cast<YogaLayoutableShadowNode*>(YGNodeGetContext(yogaNode));
 }
 
 yoga::Config& YogaLayoutableShadowNode::initializeYogaConfig(

private/react-native-fantom/tester/src/stubs/StubComponentRegistryFactory.h

@@ -16,6 +16,7 @@
 #include <react/renderer/components/text/RawTextComponentDescriptor.h>
 #include <react/renderer/components/text/SelectableParagraphComponentDescriptor.h>
 #include <react/renderer/components/text/TextComponentDescriptor.h>
+#include <react/renderer/components/view/LayoutConformanceComponentDescriptor.h>
 #include <react/renderer/components/view/ViewComponentDescriptor.h>
 
 namespace facebook::react {
@@ -27,6 +28,7 @@ inline ComponentRegistryFactory getDefaultComponentRegistryFactory()
     static auto providerRegistry = []() {
       auto providerRegistry = std::make_shared<ComponentDescriptorProviderRegistry>();
       providerRegistry->add(concreteComponentDescriptorProvider<ImageComponentDescriptor>());
+      providerRegistry->add(concreteComponentDescriptorProvider<LayoutConformanceComponentDescriptor>());
       providerRegistry->add(concreteComponentDescriptorProvider<ParagraphComponentDescriptor>());
       providerRegistry->add(concreteComponentDescriptorProvider<ScrollViewComponentDescriptor>());
       providerRegistry->add(concreteComponentDescriptorProvider<RawTextComponentDescriptor>());

scripts/cxx-api/api-snapshots/ReactAndroidFrameworksCxx.api

@@ -376,6 +376,7 @@ struct facebook::hermes::ISetEventLoopControl : public facebook::jsi::ICast {
 const char facebook::react::AndroidTextInputComponentName[];
 const char facebook::react::ImageComponentName[];
 const char facebook::react::ImageManagerKey[];
+const char facebook::react::LayoutConformanceShadowNodeComponentName[];
 const char facebook::react::ModalHostViewComponentName[];
 const char facebook::react::ParagraphComponentName[];
 const char facebook::react::RawTextComponentName[];
@@ -386,7 +387,6 @@ const char facebook::react::TextComponentName[];
 const char facebook::react::ViewComponentName[];
 const facebook::react::EventTag facebook::react::EMPTY_EVENT_TAG;
 constexpr auto facebook::react::kSimpleThreadSafeCacheSizeCap;
-constexpr char facebook::react::LayoutConformanceShadowNodeComponentName[];
 constexpr char* const facebook::react::TextLayoutManagerKey;
 constexpr facebook::react::ReactNativeVersionType facebook::react::ReactNativeVersion;
 constexpr facebook::react::Tag facebook::react::kNoTag;

scripts/cxx-api/api-snapshots/ReactAppleFrameworksCxx.api

@@ -3186,6 +3186,7 @@ struct facebook::hermes::ISetEventLoopControl : public facebook::jsi::ICast {
 
 const char facebook::react::ImageComponentName[];
 const char facebook::react::ImageManagerKey[];
+const char facebook::react::LayoutConformanceShadowNodeComponentName[];
 const char facebook::react::ModalHostViewComponentName[];
 const char facebook::react::ParagraphComponentName[];
 const char facebook::react::RawTextComponentName[];
@@ -3197,7 +3198,6 @@ const char facebook::react::TextInputComponentName[];
 const char facebook::react::ViewComponentName[];
 const facebook::react::EventTag facebook::react::EMPTY_EVENT_TAG;
 constexpr auto facebook::react::kSimpleThreadSafeCacheSizeCap;
-constexpr char facebook::react::LayoutConformanceShadowNodeComponentName[];
 constexpr char* const facebook::react::TextLayoutManagerKey;
 constexpr facebook::react::ReactNativeVersionType facebook::react::ReactNativeVersion;
 constexpr facebook::react::Tag facebook::react::kNoTag;

scripts/cxx-api/api-snapshots/ReactCommonFrameworksCxx.api

@@ -73,6 +73,7 @@ struct facebook::hermes::ISetEventLoopControl : public facebook::jsi::ICast {
 
 const char facebook::react::ImageComponentName[];
 const char facebook::react::ImageManagerKey[];
+const char facebook::react::LayoutConformanceShadowNodeComponentName[];
 const char facebook::react::ModalHostViewComponentName[];
 const char facebook::react::ParagraphComponentName[];
 const char facebook::react::RawTextComponentName[];
@@ -83,7 +84,6 @@ const char facebook::react::TextComponentName[];
 const char facebook::react::ViewComponentName[];
 const facebook::react::EventTag facebook::react::EMPTY_EVENT_TAG;
 constexpr auto facebook::react::kSimpleThreadSafeCacheSizeCap;
-constexpr char facebook::react::LayoutConformanceShadowNodeComponentName[];
 constexpr char* const facebook::react::TextLayoutManagerKey;
 constexpr facebook::react::ReactNativeVersionType facebook::react::ReactNativeVersion;
 constexpr facebook::react::Tag facebook::react::kNoTag;