Add alwaysThrottleDisappearingFallbacks Flag (#28550)

## Summary

Creates a new `alwaysThrottleDisappearingFallbacks` feature flag that
gates the changes from #26802
(instead of being controlled by `alwaysThrottleRetries`). The values of
this new flag mirror the current values of `alwaysThrottleRetries` such
that there is no behavior difference.

This additional feature flag allows us to incrementally validate the
change (arguably bug fix) from
#26802 independently from
`alwaysThrottleRetries`.

## How did you test this change?

```
$ yarn test
$ yarn flow dom-browser
$ yarn flow dom-fb
$ yarn flow fabric
```

Author: yungsters

packages/react-reconciler/src/ReactFiberCommitWork.js

@@ -40,6 +40,7 @@ import type {
 } from './ReactFiberTracingMarkerComponent';
 
 import {
+  alwaysThrottleDisappearingFallbacks,
   enableCreateEventHandleAPI,
   enableProfilerTimer,
   enableProfilerCommitHooks,
@@ -53,7 +54,6 @@ import {
   enableUseEffectEventHook,
   enableFloat,
   enableLegacyHidden,
-  alwaysThrottleRetries,
   disableStringRefs,
 } from 'shared/ReactFeatureFlags';
 import {
@@ -2896,7 +2896,7 @@ function commitMutationEffectsOnFiber(
           current !== null &&
           (current.memoizedState: SuspenseState | null) !== null;
 
-        if (alwaysThrottleRetries) {
+        if (alwaysThrottleDisappearingFallbacks) {
           if (isShowingFallback !== wasShowingFallback) {
             // A fallback is either appearing or disappearing.
             markCommitTimeOfFallback();

packages/shared/ReactFeatureFlags.js

@@ -113,6 +113,8 @@ export const enableUseEffectEventHook = __EXPERIMENTAL__;
 // (handled with an MutationObserver) instead of inline-scripts
 export const enableFizzExternalRuntime = true;
 
+export const alwaysThrottleDisappearingFallbacks = true;
+
 export const alwaysThrottleRetries = true;
 
 export const passChildrenWhenCloningPersistedNodes = false;

packages/shared/forks/ReactFeatureFlags.native-fb-dynamic.js

@@ -20,6 +20,7 @@ import typeof * as DynamicFlagsType from 'ReactNativeInternalFeatureFlags';
 // flag here but it won't be set to `true` in any of our test runs. Need to
 // update the test configuration.
 
+export const alwaysThrottleDisappearingFallbacks = __VARIANT__;
 export const alwaysThrottleRetries = __VARIANT__;
 export const consoleManagedByDevToolsDuringStrictMode = __VARIANT__;
 export const enableAsyncActions = __VARIANT__;

packages/shared/forks/ReactFeatureFlags.native-fb.js

@@ -18,6 +18,7 @@ import * as dynamicFlags from 'ReactNativeInternalFeatureFlags';
 // We destructure each value before re-exporting to avoid a dynamic look-up on
 // the exports object every time a flag is read.
 export const {
+  alwaysThrottleDisappearingFallbacks,
   alwaysThrottleRetries,
   consoleManagedByDevToolsDuringStrictMode,
   enableAsyncActions,

packages/shared/forks/ReactFeatureFlags.native-oss.js

@@ -86,6 +86,7 @@ export const enableInfiniteRenderLoopDetection = false;
 
 export const enableAsyncActions = false;
 
+export const alwaysThrottleDisappearingFallbacks = false;
 export const alwaysThrottleRetries = false;
 
 export const passChildrenWhenCloningPersistedNodes = false;

packages/shared/forks/ReactFeatureFlags.test-renderer.js

@@ -76,6 +76,7 @@ export const enableDeferRootSchedulingToMicrotask = true;
 
 export const enableAsyncActions = true;
 
+export const alwaysThrottleDisappearingFallbacks = true;
 export const alwaysThrottleRetries = true;
 
 export const passChildrenWhenCloningPersistedNodes = false;

packages/shared/forks/ReactFeatureFlags.test-renderer.native.js

@@ -77,6 +77,7 @@ export const enableDeferRootSchedulingToMicrotask = false;
 
 export const enableAsyncActions = true;
 
+export const alwaysThrottleDisappearingFallbacks = true;
 export const alwaysThrottleRetries = true;
 
 export const passChildrenWhenCloningPersistedNodes = false;

packages/shared/forks/ReactFeatureFlags.test-renderer.www.js

@@ -78,6 +78,7 @@ export const enableDeferRootSchedulingToMicrotask = true;
 
 export const enableAsyncActions = true;
 
+export const alwaysThrottleDisappearingFallbacks = false;
 export const alwaysThrottleRetries = true;
 
 export const passChildrenWhenCloningPersistedNodes = false;

packages/shared/forks/ReactFeatureFlags.www-dynamic.js

@@ -23,6 +23,7 @@ export const forceConcurrentByDefaultForTesting = __VARIANT__;
 export const enableUnifiedSyncLane = __VARIANT__;
 export const enableTransitionTracing = __VARIANT__;
 export const enableDeferRootSchedulingToMicrotask = __VARIANT__;
+export const alwaysThrottleDisappearingFallbacks = __VARIANT__;
 export const alwaysThrottleRetries = __VARIANT__;
 export const enableDO_NOT_USE_disableStrictPassiveEffect = __VARIANT__;
 export const enableUseDeferredValueInitialArg = __VARIANT__;

packages/shared/forks/ReactFeatureFlags.www.js

@@ -26,6 +26,7 @@ export const {
   enableRetryLaneExpiration,
   enableTransitionTracing,
   enableDeferRootSchedulingToMicrotask,
+  alwaysThrottleDisappearingFallbacks,
   alwaysThrottleRetries,
   enableDO_NOT_USE_disableStrictPassiveEffect,
   disableSchedulerTimeoutInWorkLoop,