Add flag to disable behavior change

packages/react-dom/src/__tests__/ReactDOMServerSelectiveHydration-test.internal.js

@@ -1067,6 +1067,7 @@ describe('ReactDOMServerSelectiveHydration', () => {
       assertLog(['Inner Mouse Enter', 'Outer Mouse Enter']);
     });
 
+    // @gate enableDeferRootSchedulingToMicrotask
     it('Outer hydrates first then Inner', async () => {
       dispatchMouseHoverEvent(innerDiv);
 

packages/react-reconciler/src/ReactFiberRootScheduler.js

@@ -11,6 +11,7 @@ import type {FiberRoot} from './ReactInternalTypes';
 import type {Lane} from './ReactFiberLane';
 import type {PriorityLevel} from 'scheduler/src/SchedulerPriorities';
 
+import {enableDeferRootSchedulingToMicrotask} from 'shared/ReactFeatureFlags';
 import {
   NoLane,
   NoLanes,
@@ -41,6 +42,7 @@ import {
   cancelCallback as Scheduler_cancelCallback,
   scheduleCallback as Scheduler_scheduleCallback,
   now,
+  ImmediatePriority,
 } from './Scheduler';
 import {
   DiscreteEventPriority,
@@ -111,14 +113,30 @@ export function ensureRootIsScheduled(root: FiberRoot): void {
       scheduleImmediateTask(processRootScheduleInMicrotask);
     }
   }
+
+  if (!enableDeferRootSchedulingToMicrotask) {
+    // While this flag is disabled, we schedule the task immediately instead
+    // of waiting for the microtask to fire.
+    // TODO: We need to land enableDeferRootSchedulingToMicrotask ASAP to
+    // unblock additional features we have planned.
+    scheduleTaskForRootDuringMicrotask(root, now());
+    if (mightHavePendingSyncWork) {
+      // Before enableDeferRootSchedulingToMicrotask, we used to flush
+      // synchronous work in a macrotask instead of a microtask. (Except for
+      // discrete events or flushSync, which will force this to run
+      // even sooner.)
+      scheduleCallback(ImmediatePriority, flushSyncCallbacks);
+    }
+  }
 }
 
 // TODO: Rename to something else. This isn't a generic callback queue anymore.
 // I only kept the existing name to reduce noise in the initial PR diff.
-export function flushSyncCallbacks() {
+export function flushSyncCallbacks(): null {
   // This is allowed to be called synchronously, but the caller should check
   // the execution context first.
   flushSyncWorkAcrossRoots_impl(false);
+  return null;
 }
 
 // TODO: Rename to something else. This isn't a generic callback queue anymore.
@@ -254,9 +272,16 @@ function processRootScheduleInMicrotask() {
     root = next;
   }
 
-  // At the end of the microtask, flush any pending synchronous work. This has
-  // to come at the end, because it does actual rendering work that might throw.
-  flushSyncCallbacks();
+  if (enableDeferRootSchedulingToMicrotask) {
+    // At the end of the microtask, flush any pending synchronous work. This has
+    // to come at the end, because it does actual rendering work that might throw.
+    flushSyncCallbacks();
+  } else {
+    // Before enableDeferRootSchedulingToMicrotask, we used to flush synchronous
+    // work in a macrotask instead of a microtask. (Except for discrete events
+    // or flushSync, which will force this to run even sooner.)
+    scheduleCallback(ImmediatePriority, flushSyncCallbacks);
+  }
 }
 
 function scheduleTaskForRootDuringMicrotask(
@@ -267,6 +292,9 @@ function scheduleTaskForRootDuringMicrotask(
   // rendering task right before we yield to the main thread. It should never be
   // called synchronously.
   //
+  // TODO: Unless enableDeferRootSchedulingToMicrotask is off. We need to land
+  // that ASAP to unblock additional features we have planned.
+  //
   // This function also never performs React work synchronously; it should
   // only schedule work to be performed later, in a separate task or microtask.
 

packages/react/src/__tests__/ReactProfiler-test.internal.js

@@ -258,7 +258,22 @@ describe(`onRender`, () => {
 
     // TODO: unstable_now is called by more places than just the profiler.
     // Rewrite this test so it's less fragile.
-    assertLog(['read current time', 'read current time', 'read current time']);
+    if (gate(flags => flags.enableDeferRootSchedulingToMicrotask)) {
+      assertLog([
+        'read current time',
+        'read current time',
+        'read current time',
+      ]);
+    } else {
+      assertLog([
+        'read current time',
+        'read current time',
+        'read current time',
+        'read current time',
+        'read current time',
+        'read current time',
+      ]);
+    }
 
     // Restore original mock
     jest.mock('scheduler', () => jest.requireActual('scheduler/unstable_mock'));

packages/shared/ReactFeatureFlags.js

@@ -52,6 +52,10 @@ export const enableSchedulerDebugging = false;
 // Need to remove didTimeout argument from Scheduler before landing
 export const disableSchedulerTimeoutInWorkLoop = false;
 
+// This will break some internal tests at Meta so we need to gate this until
+// those can be fixed.
+export const enableDeferRootSchedulingToMicrotask = true;
+
 // -----------------------------------------------------------------------------
 // Slated for removal in the future (significant effort)
 //

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

@@ -21,6 +21,7 @@ import typeof * as DynamicFlagsType from 'ReactNativeInternalFeatureFlags';
 // update the test configuration.
 
 export const enableUseRefAccessWarning = __VARIANT__;
+export const enableDeferRootSchedulingToMicrotask = __VARIANT__;
 
 // Flow magic to verify the exports of this file match the original version.
 ((((null: any): ExportsType): DynamicFlagsType): ExportsType);

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

@@ -17,7 +17,8 @@ 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 {enableUseRefAccessWarning} = dynamicFlags;
+export const {enableUseRefAccessWarning, enableDeferRootSchedulingToMicrotask} =
+  dynamicFlags;
 
 // The rest of the flags are static for better dead code elimination.
 export const enableDebugTracing = false;

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

@@ -70,6 +70,7 @@ export const enableHostSingletons = true;
 
 export const useModernStrictMode = false;
 export const enableFizzExternalRuntime = false;
+export const enableDeferRootSchedulingToMicrotask = true;
 
 // Flow magic to verify the exports of this file match the original version.
 ((((null: any): ExportsType): FeatureFlagsType): ExportsType);

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

@@ -70,6 +70,7 @@ export const enableHostSingletons = true;
 
 export const useModernStrictMode = false;
 export const enableFizzExternalRuntime = false;
+export const enableDeferRootSchedulingToMicrotask = true;
 
 // Flow magic to verify the exports of this file match the original version.
 ((((null: any): ExportsType): FeatureFlagsType): ExportsType);

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

@@ -72,6 +72,7 @@ export const enableHostSingletons = true;
 
 export const useModernStrictMode = false;
 export const enableFizzExternalRuntime = false;
+export const enableDeferRootSchedulingToMicrotask = true;
 
 // Flow magic to verify the exports of this file match the original version.
 ((((null: any): ExportsType): FeatureFlagsType): ExportsType);

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

@@ -24,6 +24,7 @@ export const enableLazyContextPropagation = __VARIANT__;
 export const enableUnifiedSyncLane = __VARIANT__;
 export const enableTransitionTracing = __VARIANT__;
 export const enableCustomElementPropertySupport = __VARIANT__;
+export const enableDeferRootSchedulingToMicrotask = __VARIANT__;
 
 // Enable this flag to help with concurrent mode debugging.
 // It logs information to the console about React scheduling, rendering, and commit phases.

packages/shared/forks/ReactFeatureFlags.www.js

@@ -28,6 +28,7 @@ export const {
   enableUnifiedSyncLane,
   enableTransitionTracing,
   enableCustomElementPropertySupport,
+  enableDeferRootSchedulingToMicrotask,
 } = dynamicFeatureFlags;
 
 // On WWW, __EXPERIMENTAL__ is used for a new modern build.

scripts/flow/xplat.js

@@ -9,4 +9,5 @@
 
 declare module 'ReactNativeInternalFeatureFlags' {
   declare export var enableUseRefAccessWarning: boolean;
+  declare export var enableDeferRootSchedulingToMicrotask: boolean;
 }