remove enableSchedulerYield flag, can rely on origin trial token instead

packages/scheduler/src/SchedulerFeatureFlags.js

@@ -14,4 +14,3 @@ export const enableIsInputPendingContinuous = false;
 export const frameYieldMs = 5;
 export const continuousYieldMs = 50;
 export const maxYieldMs = 300;
-export const enableSchedulerYield = false;

packages/scheduler/src/__tests__/SchedulerPostTask-test.js

@@ -96,9 +96,7 @@ describe('SchedulerPostTask', () => {
 
     scheduler.yield = function ({priority, signal}) {
       const id = idCounter++;
-      log(
-        `Post Task ${id} [${priority === undefined ? '<default>' : priority}]`,
-      );
+      log(`Yield ${id} [${priority === undefined ? '<default>' : priority}]`);
       const controller = signal._controller;
       let callback;
 
@@ -196,7 +194,7 @@ describe('SchedulerPostTask', () => {
       'Task 0 Fired',
       'A',
       'Yield at 5ms',
-      'Post Task 1 [user-visible]',
+      'Yield 1 [user-visible]',
     ]);
 
     runtime.flushTasks();
@@ -339,7 +337,7 @@ describe('SchedulerPostTask', () => {
 
       // The continuation should be scheduled in a separate macrotask even
       // though there's time remaining.
-      'Post Task 1 [user-visible]',
+      'Yield 1 [user-visible]',
     ]);
 
     // No time has elapsed
@@ -348,4 +346,67 @@ describe('SchedulerPostTask', () => {
     runtime.flushTasks();
     runtime.assertLog(['Task 1 Fired', 'Continuation Task']);
   });
+
+  describe('falls back to postTask for scheduling continuations when scheduler.yield is not available', () => {
+    beforeEach(() => {
+      delete global.scheduler.yield;
+    });
+
+    it('task with continuation', () => {
+      scheduleCallback(NormalPriority, () => {
+        runtime.log('A');
+        while (!Scheduler.unstable_shouldYield()) {
+          runtime.advanceTime(1);
+        }
+        runtime.log(`Yield at ${performance.now()}ms`);
+        return () => {
+          runtime.log('Continuation');
+        };
+      });
+      runtime.assertLog(['Post Task 0 [user-visible]']);
+
+      runtime.flushTasks();
+      runtime.assertLog([
+        'Task 0 Fired',
+        'A',
+        'Yield at 5ms',
+        'Post Task 1 [user-visible]',
+      ]);
+
+      runtime.flushTasks();
+      runtime.assertLog(['Task 1 Fired', 'Continuation']);
+    });
+
+    it('yielding continues in a new task regardless of how much time is remaining', () => {
+      scheduleCallback(NormalPriority, () => {
+        runtime.log('Original Task');
+        runtime.log('shouldYield: ' + shouldYield());
+        runtime.log('Return a continuation');
+        return () => {
+          runtime.log('Continuation Task');
+        };
+      });
+      runtime.assertLog(['Post Task 0 [user-visible]']);
+
+      runtime.flushTasks();
+      runtime.assertLog([
+        'Task 0 Fired',
+        'Original Task',
+        // Immediately before returning a continuation, `shouldYield` returns
+        // false, which means there must be time remaining in the frame.
+        'shouldYield: false',
+        'Return a continuation',
+
+        // The continuation should be scheduled in a separate macrotask even
+        // though there's time remaining.
+        'Post Task 1 [user-visible]',
+      ]);
+
+      // No time has elapsed
+      expect(performance.now()).toBe(0);
+
+      runtime.flushTasks();
+      runtime.assertLog(['Task 1 Fired', 'Continuation Task']);
+    });
+  });
 });

packages/scheduler/src/forks/SchedulerFeatureFlags.www-dynamic.js

@@ -12,4 +12,3 @@
 // with the __VARIANT__ set to `true`, and once set to `false`.
 
 export const enableProfiling = __VARIANT__;
-export const enableSchedulerYield = __VARIANT__;

packages/scheduler/src/forks/SchedulerFeatureFlags.www.js

@@ -7,17 +7,13 @@
  * @flow
  */
 
-const {
-  enableProfiling: enableProfilingFeatureFlag,
-  enableSchedulerYield: enableSchedulerYieldFeatureFlag,
-} =
+const {enableProfiling: enableProfilingFeatureFlag} =
   // $FlowFixMe[cannot-resolve-module]
   require('SchedulerFeatureFlags');
 
 export const enableSchedulerDebugging = true;
 export const enableProfiling: boolean =
   __PROFILE__ && enableProfilingFeatureFlag;
-export const enableSchedulerYield = enableSchedulerYieldFeatureFlag;
 export const enableIsInputPending = true;
 export const enableIsInputPendingContinuous = true;
 export const frameYieldMs = 5;

packages/scheduler/src/forks/SchedulerPostTask.js

@@ -9,8 +9,6 @@
 
 import type {PriorityLevel} from '../SchedulerPriorities';
 
-import {enableSchedulerYield} from '../SchedulerFeatureFlags';
-
 declare class TaskController {
   constructor(priority?: string): TaskController;
   signal: mixed;
@@ -149,7 +147,7 @@ function runTask<T>(
         continuation,
       );
 
-      if (enableSchedulerYield && scheduler.yield !== undefined) {
+      if (scheduler.yield !== undefined) {
         scheduler
           .yield(continuationOptions)
           .then(nextTask)