Add useTraitHiddenOnIOS feature flag

## Summary:

Mirrors useTraitHiddenOnAndroid from #54112. Lets iOS apps opt out of
the `Trait::Hidden` slice-skip; Android can already opt in. Default is
`true`, preserving today's iOS behavior.

What the optimization does

D22134220 (2020, "Fabric: display: none nodes do not create views
anymore") added `Trait::Hidden` and a slice-skip in
`sliceChildShadowNodeViewPairs` that filters out subtrees whose Yoga
display is None. On iOS the diff then issues REMOVE + DELETE for the
entire subtree — invisible content stops costing anything.

For most UIs this is a clean win.

Where it bites

The optimization assumes that `display: none` transitions are rare.
That breaks for a specific pattern: **a custom host component mounted
under `<Suspense>`**.

Each suspend → resume cycle, Suspense flips display between `flex` and
`none`. With the optimization on, every cycle:

- tears down the entire subtree of UIViews,
- drops per-instance native state — measurement caches, scroll
  position, animation drivers, anything internal to the host
  component,
- re-runs `init`/`dealloc` and rebuilds the subtree on resume.

Suspend/resume ends up heavier than a fresh mount, and any state the
component held disappears between renders.

What the flag does

**It re-activates an existing code path. Nothing in the mounting
layer is new.**

The hide-via-`UIView.hidden` wiring shipped in D8460108 (June 2018,
"Fabric: Default support of displayType and layoutDirection
layout...") and has lived in `UIView+ComponentViewProtocol
updateLayoutMetrics:` ever since — a few lines below the slice
consumer in the same file. For two years it was the only iOS path;
the 2020 slice-skip didn't replace it, it just made it unreachable
in the common case.

Setting the flag to `false` lets Hidden shadow nodes pass through
the slice. The differ emits an `UPDATE_LAYOUT_METRICS` mutation
with `displayType == None`, and the 2018 wiring picks it up:
`self.hidden = YES` on the underlying UIView. Same view, hidden in
place.

Defaults

- `useTraitHiddenOnIOS = true` — keeps the iOS behavior introduced
  in 2020.
- `useTraitHiddenOnAndroid = false` — keeps the Android behavior,
  which never adopted the slice-skip.

Both flags share the same semantic ("use the optimization"); the
defaults encode each platform's pre-flag behavior. Flipping either
default is out of scope.

## Changelog:

[IOS] [ADDED] - useTraitHiddenOnIOS feature flag to opt out of the `display: none` slice-skip optimization

## Test Plan:

No new tests. `StackingContextTest` exercises the slice-skip path and
passes unchanged with the flag at its default `true`. Manually
flipping the flag to `false` produces the existing Android branch's
expected view tree (8 views, Hidden subtrees preserved with
`self.hidden = YES`).

Author: SudoPlz

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<<94a1d6bbc5595c495a2c54851a9c5b5b>>
+ * @generated SignedSource<<a03d363d13ff18dd87d15dc223c9c32d>>
  */
 
 /**
@@ -396,6 +396,12 @@ public object ReactNativeFeatureFlags {
   @JvmStatic
   public fun useSilenceErrorSMMViewNotFound(): Boolean = accessor.useSilenceErrorSMMViewNotFound()
 
+  /**
+   * iOS only. When true (default), shadow nodes carrying ShadowNodeTraits::Trait::Hidden are filtered out of the mounting slice. When false, those nodes stay in the slice and are hidden via UIView.hidden = YES in updateLayoutMetrics:.
+   */
+  @JvmStatic
+  public fun useTraitHiddenOnIOS(): Boolean = accessor.useTraitHiddenOnIOS()
+
   /**
    * In Bridgeless mode, should legacy NativeModules use the TurboModule system?
    */

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<<4e19cdce9383c287c849b912fa74586c>>
+ * @generated SignedSource<<b3ddcd5c53e3fec3c0397dbeaa2a3b0c>>
  */
 
 /**
@@ -81,6 +81,7 @@ internal class ReactNativeFeatureFlagsCxxAccessor : ReactNativeFeatureFlagsAcces
   private var useRawPropsJsiValueCache: Boolean? = null
   private var useShadowNodeStateOnCloneCache: Boolean? = null
   private var useSilenceErrorSMMViewNotFoundCache: Boolean? = null
+  private var useTraitHiddenOnIOSCache: Boolean? = null
   private var useTurboModuleInteropCache: Boolean? = null
   private var useTurboModulesCache: Boolean? = null
   private var virtualViewPrerenderRatioCache: Double? = null
@@ -635,6 +636,15 @@ internal class ReactNativeFeatureFlagsCxxAccessor : ReactNativeFeatureFlagsAcces
     return cached
   }
 
+  override fun useTraitHiddenOnIOS(): Boolean {
+    var cached = useTraitHiddenOnIOSCache
+    if (cached == null) {
+      cached = ReactNativeFeatureFlagsCxxInterop.useTraitHiddenOnIOS()
+      useTraitHiddenOnIOSCache = cached
+    }
+    return cached
+  }
+
   override fun useTurboModuleInterop(): Boolean {
     var cached = useTurboModuleInteropCache
     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<<2a525ef08b04eb1c244badb8cc865e4d>>
+ * @generated SignedSource<<1234d5c7c6bb64451618817387318d13>>
  */
 
 /**
@@ -150,6 +150,8 @@ public object ReactNativeFeatureFlagsCxxInterop {
 
   @DoNotStrip @JvmStatic public external fun useSilenceErrorSMMViewNotFound(): Boolean
 
+  @DoNotStrip @JvmStatic public external fun useTraitHiddenOnIOS(): Boolean
+
   @DoNotStrip @JvmStatic public external fun useTurboModuleInterop(): Boolean
 
   @DoNotStrip @JvmStatic public external fun useTurboModules(): 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<<58b3101bd16b63268440523cdee7ae93>>
+ * @generated SignedSource<<3255db6cb7c71d866991568dc632321f>>
  */
 
 /**
@@ -145,6 +145,8 @@ public open class ReactNativeFeatureFlagsDefaults : ReactNativeFeatureFlagsProvi
 
   override fun useSilenceErrorSMMViewNotFound(): Boolean = false
 
+  override fun useTraitHiddenOnIOS(): Boolean = true
+
   override fun useTurboModuleInterop(): Boolean = false
 
   override fun useTurboModules(): 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<<3c639d5edf52fc3feee65388b5ac3ed7>>
+ * @generated SignedSource<<6f23f8fd399adf79814bb8f2edc29841>>
  */
 
 /**
@@ -85,6 +85,7 @@ internal class ReactNativeFeatureFlagsLocalAccessor : ReactNativeFeatureFlagsAcc
   private var useRawPropsJsiValueCache: Boolean? = null
   private var useShadowNodeStateOnCloneCache: Boolean? = null
   private var useSilenceErrorSMMViewNotFoundCache: Boolean? = null
+  private var useTraitHiddenOnIOSCache: Boolean? = null
   private var useTurboModuleInteropCache: Boolean? = null
   private var useTurboModulesCache: Boolean? = null
   private var virtualViewPrerenderRatioCache: Double? = null
@@ -700,6 +701,16 @@ internal class ReactNativeFeatureFlagsLocalAccessor : ReactNativeFeatureFlagsAcc
     return cached
   }
 
+  override fun useTraitHiddenOnIOS(): Boolean {
+    var cached = useTraitHiddenOnIOSCache
+    if (cached == null) {
+      cached = currentProvider.useTraitHiddenOnIOS()
+      accessedFeatureFlags.add("useTraitHiddenOnIOS")
+      useTraitHiddenOnIOSCache = cached
+    }
+    return cached
+  }
+
   override fun useTurboModuleInterop(): Boolean {
     var cached = useTurboModuleInteropCache
     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<<0a4b05562ed8b30b7d4dabd5c8680d6d>>
+ * @generated SignedSource<<2877a4acf03951884d1d663fdf00b9f8>>
  */
 
 /**
@@ -145,6 +145,8 @@ public interface ReactNativeFeatureFlagsProvider {
 
   @DoNotStrip public fun useSilenceErrorSMMViewNotFound(): Boolean
 
+  @DoNotStrip public fun useTraitHiddenOnIOS(): Boolean
+
   @DoNotStrip public fun useTurboModuleInterop(): Boolean
 
   @DoNotStrip public fun useTurboModules(): 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<<64b1d9667e6869ea4ba406222b307464>>
+ * @generated SignedSource<<c3ce57d947ffe2b5b404acd4a8527ae0>>
  */
 
 /**
@@ -405,6 +405,12 @@ class ReactNativeFeatureFlagsJavaProvider
     return method(javaProvider_);
   }
 
+  bool useTraitHiddenOnIOS() override {
+    static const auto method =
+        getReactNativeFeatureFlagsProviderJavaClass()->getMethod<jboolean()>("useTraitHiddenOnIOS");
+    return method(javaProvider_);
+  }
+
   bool useTurboModuleInterop() override {
     static const auto method =
         getReactNativeFeatureFlagsProviderJavaClass()->getMethod<jboolean()>("useTurboModuleInterop");
@@ -738,6 +744,11 @@ bool JReactNativeFeatureFlagsCxxInterop::useSilenceErrorSMMViewNotFound(
   return ReactNativeFeatureFlags::useSilenceErrorSMMViewNotFound();
 }
 
+bool JReactNativeFeatureFlagsCxxInterop::useTraitHiddenOnIOS(
+    facebook::jni::alias_ref<JReactNativeFeatureFlagsCxxInterop> /*unused*/) {
+  return ReactNativeFeatureFlags::useTraitHiddenOnIOS();
+}
+
 bool JReactNativeFeatureFlagsCxxInterop::useTurboModuleInterop(
     facebook::jni::alias_ref<JReactNativeFeatureFlagsCxxInterop> /*unused*/) {
   return ReactNativeFeatureFlags::useTurboModuleInterop();
@@ -972,6 +983,9 @@ void JReactNativeFeatureFlagsCxxInterop::registerNatives() {
       makeNativeMethod(
         "useSilenceErrorSMMViewNotFound",
         JReactNativeFeatureFlagsCxxInterop::useSilenceErrorSMMViewNotFound),
+      makeNativeMethod(
+        "useTraitHiddenOnIOS",
+        JReactNativeFeatureFlagsCxxInterop::useTraitHiddenOnIOS),
       makeNativeMethod(
         "useTurboModuleInterop",
         JReactNativeFeatureFlagsCxxInterop::useTurboModuleInterop),

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<<073595986845439fdcb30edc52dadc30>>
+ * @generated SignedSource<<f4d8c8c96e3d7075dc71b8ac048fc7e0>>
  */
 
 /**
@@ -213,6 +213,9 @@ class JReactNativeFeatureFlagsCxxInterop
   static bool useSilenceErrorSMMViewNotFound(
     facebook::jni::alias_ref<JReactNativeFeatureFlagsCxxInterop>);
 
+  static bool useTraitHiddenOnIOS(
+    facebook::jni::alias_ref<JReactNativeFeatureFlagsCxxInterop>);
+
   static bool useTurboModuleInterop(
     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<<eb8488e034b281dc5cf384aa48b0ecf0>>
+ * @generated SignedSource<<bd604e84ed03731ac48b2c0f41b36089>>
  */
 
 /**
@@ -270,6 +270,10 @@ bool ReactNativeFeatureFlags::useSilenceErrorSMMViewNotFound() {
   return getAccessor().useSilenceErrorSMMViewNotFound();
 }
 
+bool ReactNativeFeatureFlags::useTraitHiddenOnIOS() {
+  return getAccessor().useTraitHiddenOnIOS();
+}
+
 bool ReactNativeFeatureFlags::useTurboModuleInterop() {
   return getAccessor().useTurboModuleInterop();
 }

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<<7b1258f9970366f0c068f28e2c16ce12>>
+ * @generated SignedSource<<8dc801da5ad5fd085dcedb245f7dee08>>
  */
 
 /**
@@ -344,6 +344,11 @@ class ReactNativeFeatureFlags {
    */
   RN_EXPORT static bool useSilenceErrorSMMViewNotFound();
 
+  /**
+   * iOS only. When true (default), shadow nodes carrying ShadowNodeTraits::Trait::Hidden are filtered out of the mounting slice. When false, those nodes stay in the slice and are hidden via UIView.hidden = YES in updateLayoutMetrics:.
+   */
+  RN_EXPORT static bool useTraitHiddenOnIOS();
+
   /**
    * In Bridgeless mode, should legacy NativeModules use the TurboModule system?
    */