Add animatedDeferStartOfTimingAnimations feature flag

Summary:
Add the `animatedDeferStartOfTimingAnimations` common (native + JS) feature flag (default off). On its own this is a no-op; it gates deferring the start of native-driven timing animations to the first rendered frame, implemented in the following diffs.

The flag is defined as a `common` flag (not JS-only) so the same value can be read from both the JS Animated layer and the native C++ animation code. This diff includes all the generated accessors, regenerated via `yarn featureflags --update`: Kotlin (`ReactNativeFeatureFlags*.kt`), C++ (`ReactNativeFeatureFlags*.h`/`.cpp` and the JNI interop), the JS `ReactNativeFeatureFlags.js`, and the native module spec.

Changelog: [Internal]

Reviewed By: rubennorte

Differential Revision: D106825629

Author: zeyap

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

@@ -992,6 +992,17 @@ const definitions: FeatureFlagDefinitions = {
 
   jsOnly: {
     ...testDefinitions.jsOnly,
+    animatedDeferStartOfTimingAnimations: {
+      defaultValue: false,
+      metadata: {
+        dateAdded: '2026-05-26',
+        description:
+          'When enabled, the JS Animated layer defers the start of native-driven timing animations to the first rendered frame and re-anchors timing to prevent skipping initial frames when the UI thread is busy with layout work.',
+        expectedReleaseValue: true,
+        purpose: 'experimentation',
+      },
+      ossReleaseStage: 'none',
+    },
     animatedShouldDebounceQueueFlush: {
       defaultValue: false,
       metadata: {

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<<77b178e216aa86a309f46cbf661d9122>>
+ * @generated SignedSource<<4cc0d1231e555cb7f8ca416d5439c2fd>>
  * @flow strict
  * @noformat
  */
@@ -29,6 +29,7 @@ import {
 
 export type ReactNativeFeatureFlagsJsOnly = $ReadOnly<{
   jsOnlyTestFlag: Getter<boolean>,
+  animatedDeferStartOfTimingAnimations: Getter<boolean>,
   animatedShouldDebounceQueueFlush: Getter<boolean>,
   animatedShouldSyncValueBeforeStartCallback: Getter<boolean>,
   animatedShouldUseSingleOp: Getter<boolean>,
@@ -142,6 +143,11 @@ export type ReactNativeFeatureFlags = $ReadOnly<{
  */
 export const jsOnlyTestFlag: Getter<boolean> = createJavaScriptFlagGetter('jsOnlyTestFlag', false);
 
+/**
+ * When enabled, the JS Animated layer defers the start of native-driven timing animations to the first rendered frame and re-anchors timing to prevent skipping initial frames when the UI thread is busy with layout work.
+ */
+export const animatedDeferStartOfTimingAnimations: Getter<boolean> = createJavaScriptFlagGetter('animatedDeferStartOfTimingAnimations', false);
+
 /**
  * Enables an experimental flush-queue debouncing in Animated.js.
  */