VirtualView: Create Activity Experiment (#53488)

Summary:
Pull Request resolved: #53488

Creates a new feature flag to experiment with `Activity` in `VirtualView`.

The feature flag enables the following treatments:

- `no-activity` is the same as what we currently do — no `Activity` and we render `null` for hidden elements.
- `activity-without-mode` wraps the children in `Activity` but does not set `mode` and still renders `null` for hidden elements.
- `activity-with-hidden-mode` wraps the children in `Activity` and sets `mode="hidden"` and continues providing `children` (not `null`) for hidden elements.

Changelog:
[Internal]

Reviewed By: rickhanlonii

Differential Revision: D81149561

fbshipit-source-id: ea2c319139962de30836d80a2492d8147cbe82ba

Author: yungsters

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

@@ -914,6 +914,16 @@ const definitions: FeatureFlagDefinitions = {
       },
       ossReleaseStage: 'none',
     },
+    virtualViewActivityBehavior: {
+      defaultValue: 'no-activity',
+      metadata: {
+        dateAdded: '2025-08-27',
+        description: 'Changes whether and how `VirtualView` uses `Activity`.',
+        expectedReleaseValue: true,
+        purpose: 'experimentation',
+      },
+      ossReleaseStage: 'none',
+    },
   },
 };
 

packages/react-native/src/private/components/virtualview/VirtualView.js

@@ -14,11 +14,13 @@ import type {HostInstance} from '../../types/HostInstance';
 import type {NativeModeChangeEvent} from './VirtualViewNativeComponent';
 
 import StyleSheet from '../../../../Libraries/StyleSheet/StyleSheet';
+import * as ReactNativeFeatureFlags from '../../featureflags/ReactNativeFeatureFlags';
 import VirtualViewExperimentalNativeComponent from './VirtualViewExperimentalNativeComponent';
 import VirtualViewNativeComponent from './VirtualViewNativeComponent';
 import nullthrows from 'nullthrows';
 import * as React from 'react';
-import {startTransition, useState} from 'react';
+// $FlowFixMe[missing-export]
+import {startTransition, unstable_Activity as Activity, useState} from 'react';
 
 // @see VirtualViewNativeComponent
 export enum VirtualViewMode {
@@ -140,7 +142,17 @@ function createVirtualView(
             : style
         }
         onModeChange={handleModeChange}>
-        {isHidden ? null : children}
+        {
+          match (ReactNativeFeatureFlags.virtualViewActivityBehavior()) {
+            'activity-without-mode' =>
+              <Activity>{isHidden ? null : children}</Activity>,
+            'activity-with-hidden-mode' =>
+              <Activity mode={isHidden ? 'hidden' : 'visible'}>
+                {children}
+              </Activity>,
+            'no-activity' | _ => isHidden ? null : children,
+          }
+        }
       </NativeComponent>
     );
   }

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<<276b1579eef10e63c77752017b97985f>>
+ * @generated SignedSource<<85a053d6a79240a7b4e181d3054ecfd3>>
  * @flow strict
  * @noformat
  */
@@ -41,6 +41,7 @@ export type ReactNativeFeatureFlagsJsOnly = $ReadOnly<{
   shouldUseAnimatedObjectForTransform: Getter<boolean>,
   shouldUseRemoveClippedSubviewsAsDefaultOnIOS: Getter<boolean>,
   shouldUseSetNativePropsInFabric: Getter<boolean>,
+  virtualViewActivityBehavior: Getter<string>,
 }>;
 
 export type ReactNativeFeatureFlagsJsOnlyOverrides = OverridesFor<ReactNativeFeatureFlagsJsOnly>;
@@ -184,6 +185,11 @@ export const shouldUseRemoveClippedSubviewsAsDefaultOnIOS: Getter<boolean> = cre
  */
 export const shouldUseSetNativePropsInFabric: Getter<boolean> = createJavaScriptFlagGetter('shouldUseSetNativePropsInFabric', true);
 
+/**
+ * Changes whether and how `VirtualView` uses `Activity`.
+ */
+export const virtualViewActivityBehavior: Getter<string> = createJavaScriptFlagGetter('virtualViewActivityBehavior', "no-activity");
+
 /**
  * Common flag for testing. Do NOT modify.
  */

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

@@ -61,6 +61,7 @@ export function createJavaScriptFlagGetter<
     configName,
     () => {
       accessedFeatureFlags.add(configName);
+      // $FlowFixMe[incompatible-type] - `defaultValue` is not refined.
       return overrides?.[configName]?.(defaultValue);
     },
     defaultValue,