Scheduler internals

Summary

React Scheduler (and browser scheduler.postTask) coordinate with isInputPending — cooperative multitasking. shouldYield ends slice. Not OS thread scheduler — still single main thread. Understanding helps debug why transitions defer but clicks don't.

Code example

if ('scheduler' in globalThis && 'postTask' in scheduler) {
  scheduler.postTask(() => heavyWork(), { priority: 'background' });
} else {
  requestIdleCallback(() => heavyWork());
}

Key points

• React Scheduler (and browser scheduler.postTask) coordinate with isInputPending — cooperative multitasking.
• shouldYield ends slice.
• Not OS thread scheduler — still single main thread.
• Understanding helps debug why transitions defer but clicks don't.

Further reading

• scheduler.postTask
• requestIdleCallback

Related concepts

• Priority inversion in async code
• Concurrent rendering tearing