Add animatedDeferStartOfTimingAnimations feature flag (#57004)

Summary:
Pull Request resolved: #57004

Add the `animatedDeferStartOfTimingAnimations` JS only 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 diff.

Changelog: [Internal]

Reviewed By: rubennorte

Differential Revision: D106825629

fbshipit-source-id: a7d5d209a89299b11b3976f44bbf89a7fae0ae49

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<<9ea39238fb9e7a4fd17f5c6a4f557e8c>>
+ * @generated SignedSource<<d9146c8c9b0c602e4cedb99e6d91558e>>
  * @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>,
@@ -143,6 +144,11 @@ 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.
  */