are relevant if the browser supports idle
  // callbacks natively. Nevertheless, run them without native idle support to
  // make sure the fallback keeps working.
  function withoutNativeIdleCallbacks(func) {
    const oldRIC = window.requestIdleCallback;
    try {
      window.requestIdleCallback = undefined;
      func();
    } finally {
      window.requestIdleCallback = oldRIC;
    }
  }

  function withMockedScheduling(func) {
    withFakeWindowPerformanceNow(function() {
      withoutNativeIdleCallba