Add back disableSchedulerTimeoutInWorkLoop flag (facebook#20482)

* Add back enableSchedulerTimeoutInWorkLoop flag

* Nvm, keep it as disableSchedulerTimeoutInWorkLoop

Author: rickhanlonii

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

@@ -31,6 +31,7 @@ import {
   enableDebugTracing,
   enableSchedulingProfiler,
   enableScopeAPI,
+  disableSchedulerTimeoutInWorkLoop,
 } from 'shared/ReactFeatureFlags';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
 import invariant from 'shared/invariant';
@@ -767,7 +768,7 @@ 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) {
   if (enableProfilerTimer && enableProfilerNestedUpdatePhase) {
     resetNestedUpdateFlag();
   }
@@ -811,6 +812,18 @@ 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

@@ -33,6 +33,7 @@ import {
   enableSchedulingProfiler,
   enableScopeAPI,
   enableDoubleInvokingEffects,
+  disableSchedulerTimeoutInWorkLoop,
 } from 'shared/ReactFeatureFlags';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
 import invariant from 'shared/invariant';
@@ -775,7 +776,7 @@ 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) {
   if (enableProfilerTimer && enableProfilerNestedUpdatePhase) {
     resetNestedUpdateFlag();
   }
@@ -819,6 +820,18 @@ 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 (