Repository: facebook/react
Pull request: Implement requestPaint in the actual scheduler

When implementing passive effects we did a pretty massive oversight. While the passive effect is scheduled into its own scheduler task, the scheduler doesn't always yield to the browser if it has time left. That means that if you have a fast commit phase, it might try to squeeze in the passive effects in the same frame but those then might end being very heavy.

We had `requestPaint()` for this but that was only implemented for the `isInputPending` experiment. It wasn't thought we needed it for the regular scheduler because it yields "every frame" anyway - but it doesn't yield every task. While the `isInputPending` experiment showed that it wasn't actually any significant impact, and it was better to keep shorter yield time anyway. Which is why we deleted the code. Whatever small win it did see in some cases might have been actually due to this issue rather than anything to do with `isInputPending` at all.

As you can see in https://github.com/facebook/react/pull/31782 we do have this implemented in the mock scheduler and a lot of behavior that we assert assumes that this works.

So this just implements yielding after `requestPaint` is called.

Before:

<img width="1023" alt="Screenshot 2024-12-14 at 3 40 24 PM" src="https://github.com/user-attachments/assets/d60f4bb2-c8f8-4f91-a402-9ac25b278450" />

After:

<img width="1108" alt="Screenshot 2024-12-14 at 3 41 25 PM" src="https://github.com/user-attachments/assets/170cdb90-a049-436f-9501-be3fb9bc04ca" />

Notice how in after the native task is split into two. It might not always actually paint and the native scheduler might make the same mistake and think it has enough time left but it's at least less likely to.

We do have another way to do this. When we yield a continuation we also yield to the native browser. This is to enable the Suspense Optimization (currently disabled) to work. We could do the same for passive effects and, in fact, I have a branch that does but because that requires a lot more tests to be fixed it's a lot more invasive of a change. The nice thing about this approach is that this is not even running in tests at all and the tests we do have assert that this is the behavior already. 😬

Changed files:
filename: packages/scheduler/src/__tests__/Scheduler-test.js
patch: @@ -183,7 +183,7 @@ describe('SchedulerBrowser', () => {
   it('task with continuation', () => {
     scheduleCallback(NormalPriority, () => {
       runtime.log('Task');
-      // Request paint so that we yield at the end of the frame interval
+      // Request paint so that we yield immediately
       requestPaint();
       while (!Scheduler.unstable_shouldYield()) {
         runtime.advanceTime(1);
@@ -199,7 +199,7 @@ describe('SchedulerBrowser', () => {
     runtime.assertLog([
       'Message Event',
       'Task',
-      gate(flags => (flags.www ? 'Yield at 10ms' : 'Yield at 5ms')),
+      'Yield at 0ms',
       'Post Message',
     ]);
 

filename: packages/scheduler/src/forks/Scheduler.js
patch: @@ -93,6 +93,8 @@ var isPerformingWork = false;
 var isHostCallbackScheduled = false;
 var isHostTimeoutScheduled = false;
 
+var needsPaint = false;
+
 // Capture local references to native APIs, in case a polyfill overrides them.
 const localSetTimeout = typeof setTimeout === 'function' ? setTimeout : null;
 const localClearTimeout =
@@ -456,6 +458,10 @@ let frameInterval = frameYieldMs;
 let startTime = -1;
 
 function shouldYieldToHost(): boolean {
+  if (needsPaint) {
+    // Yield now.
+    return true;
+  }
   const timeElapsed = getCurrentTime() - startTime;
   if (timeElapsed < frameInterval) {
     // The main thread has only been blocked for a really short amount of time;
@@ -466,7 +472,9 @@ function shouldYieldToHost(): boolean {
   return true;
 }
 
-function requestPaint() {}
+function requestPaint() {
+  needsPaint = true;
+}
 
 function forceFrameRate(fps: number) {
   if (fps < 0 || fps > 125) {
@@ -486,6 +494,7 @@ function forceFrameRate(fps: number) {
 }
 
 const performWorkUntilDeadline = () => {
+  needsPaint = false;
   if (isMessageLoopRunning) {
     const currentTime = getCurrentTime();
     // Keep track of the start time so we can measure how long the main thread