Add feature flag to disable scheduler timeout in work loop

packages/react-reconciler/src/ReactFiberWorkLoop.new.js

@@ -30,6 +30,7 @@ import {
   enableSchedulingProfiler,
   enableScopeAPI,
   skipUnmountedBoundaries,
+  disableSchedulerTimeoutInWorkLoop,
 } from 'shared/ReactFeatureFlags';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
 import invariant from 'shared/invariant';
@@ -750,7 +751,7 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
 
 // This is the entry point for every concurrent task, i.e. anything that
 // goes through Scheduler.
-function performConcurrentWorkOnRoot(root) {
+function performConcurrentWorkOnRoot(root, didTimeout) {
   // Since we know we're in a React event, we can clear the current
   // event time. The next update will compute a new event time.
   currentEventTime = NoTimestamp;
@@ -790,6 +791,18 @@ function performConcurrentWorkOnRoot(root) {
     return null;
   }
 
+  // TODO: We only check `didTimeout` defensively, to account for a Scheduler
+  // bug we're still investigating. Once the bug in Scheduler is fixed,
+  // we can remove this, since we track expiration ourselves.
+  if (!disableSchedulerTimeoutInWorkLoop && didTimeout) {
+    // Something expired. Flush synchronously until there's no expired
+    // work left.
+    markRootExpired(root, lanes);
+    // This will schedule a synchronous callback.
+    ensureRootIsScheduled(root, now());
+    return null;
+  }
+
   let exitStatus = renderRootConcurrent(root, lanes);
 
   if (

packages/react-reconciler/src/ReactFiberWorkLoop.old.js

@@ -30,6 +30,7 @@ import {
   enableSchedulingProfiler,
   enableScopeAPI,
   skipUnmountedBoundaries,
+  disableSchedulerTimeoutInWorkLoop,
 } from 'shared/ReactFeatureFlags';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
 import invariant from 'shared/invariant';
@@ -738,7 +739,7 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
 
 // This is the entry point for every concurrent task, i.e. anything that
 // goes through Scheduler.
-function performConcurrentWorkOnRoot(root) {
+function performConcurrentWorkOnRoot(root, didTimeout) {
   // Since we know we're in a React event, we can clear the current
   // event time. The next update will compute a new event time.
   currentEventTime = NoTimestamp;
@@ -778,6 +779,18 @@ function performConcurrentWorkOnRoot(root) {
     return null;
   }
 
+  // TODO: We only check `didTimeout` defensively, to account for a Scheduler
+  // bug we're still investigating. Once the bug in Scheduler is fixed,
+  // we can remove this, since we track expiration ourselves.
+  if (!disableSchedulerTimeoutInWorkLoop && didTimeout) {
+    // Something expired. Flush synchronously until there's no expired
+    // work left.
+    markRootExpired(root, lanes);
+    // This will schedule a synchronous callback.
+    ensureRootIsScheduled(root, now());
+    return null;
+  }
+
   let exitStatus = renderRootConcurrent(root, lanes);
 
   if (

packages/shared/ReactFeatureFlags.js

@@ -138,3 +138,5 @@ export const enablePassiveEventIntervention = true;
 export const disableOnScrollBubbling = true;
 
 export const enableEagerRootListeners = true;
+
+export const disableSchedulerTimeoutInWorkLoop = false;

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

@@ -52,6 +52,7 @@ export const decoupleUpdatePriorityFromScheduler = false;
 export const enableDiscreteEventFlushingChange = false;
 export const enablePassiveEventIntervention = true;
 export const enableEagerRootListeners = true;
+export const disableSchedulerTimeoutInWorkLoop = false;
 
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars

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

@@ -51,6 +51,7 @@ export const decoupleUpdatePriorityFromScheduler = false;
 export const enableDiscreteEventFlushingChange = false;
 export const enablePassiveEventIntervention = true;
 export const enableEagerRootListeners = true;
+export const disableSchedulerTimeoutInWorkLoop = false;
 
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars

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

@@ -51,6 +51,7 @@ export const decoupleUpdatePriorityFromScheduler = false;
 export const enableDiscreteEventFlushingChange = false;
 export const enablePassiveEventIntervention = true;
 export const enableEagerRootListeners = true;
+export const disableSchedulerTimeoutInWorkLoop = false;
 
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars

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

@@ -50,6 +50,7 @@ export const decoupleUpdatePriorityFromScheduler = false;
 export const enableDiscreteEventFlushingChange = false;
 export const enablePassiveEventIntervention = true;
 export const enableEagerRootListeners = true;
+export const disableSchedulerTimeoutInWorkLoop = false;
 
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars

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

@@ -51,6 +51,7 @@ export const decoupleUpdatePriorityFromScheduler = false;
 export const enableDiscreteEventFlushingChange = false;
 export const enablePassiveEventIntervention = true;
 export const enableEagerRootListeners = true;
+export const disableSchedulerTimeoutInWorkLoop = false;
 
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars

packages/shared/forks/ReactFeatureFlags.testing.js

@@ -51,6 +51,7 @@ export const decoupleUpdatePriorityFromScheduler = false;
 export const enableDiscreteEventFlushingChange = false;
 export const enablePassiveEventIntervention = true;
 export const enableEagerRootListeners = true;
+export const disableSchedulerTimeoutInWorkLoop = false;
 
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars

packages/shared/forks/ReactFeatureFlags.testing.www.js

@@ -51,6 +51,7 @@ export const decoupleUpdatePriorityFromScheduler = false;
 export const enableDiscreteEventFlushingChange = true;
 export const enablePassiveEventIntervention = true;
 export const enableEagerRootListeners = true;
+export const disableSchedulerTimeoutInWorkLoop = false;
 
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars

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

@@ -49,3 +49,4 @@ export const replayFailedUnitOfWorkWithInvokeGuardedCallback = __DEV__;
 // to __VARIANT__.
 export const enableTrustedTypesIntegration = false;
 export const disableSchedulerTimeoutBasedOnReactExpirationTime = false;
+export const disableSchedulerTimeoutInWorkLoop = __VARIANT__;

packages/shared/forks/ReactFeatureFlags.www.js

@@ -30,6 +30,7 @@ export const {
   enablePassiveEventIntervention,
   disableOnScrollBubbling,
   enableEagerRootListeners,
+  disableSchedulerTimeoutInWorkLoop,
 } = dynamicFeatureFlags;
 
 // On WWW, __EXPERIMENTAL__ is used for a new modern build.