https://github.com/facebook/react/blob/30b47103d4354d9187dc0f1fb804855a5208ca9f/packages/scheduler/src/Scheduler.js#L180-L185
isHostTimeoutScheduled = false;
  advanceTimers(currentTime);
  if (!isHostCallbackScheduled) {
    if (peek(taskQueue) !== null) {
      isHostCallbackScheduled = true;
      requestHostCallback(flushWork);
    } else {
      const firstTimer = peek(timerQueue);
      if (firstTimer !== null) {
        requestHostTimeout(handleTimeout, firstTimer.startTime - currentTime);
      }
    }
  }
}
function flushWork(hasTimeRemaining, initialTime) {
  if (enableProfiling) {
    markSchedulerUnsuspended(initialTime);
  }
  
  isHostCallbackScheduled = false;
  if (isHostTimeoutScheduled) {
    
    isHostTimeoutScheduled = false;
    cancelHostTimeout();