mozilla-release/testing/web-platform/tests/event-timing/resources/event-timing-crossiframe-childframe.html

<!DOCType html>
<html>
<script src=event-timing-support.js></script>
<button id='button_child_frame'>Generate a 'click' event</button>
<img src=slow-image.py>
<script>
  const clickTimeMin = performance.now();
  clickAndBlockMain('button_child_frame');
  const processingStartMin = performance.now();
  const observerPromise = new Promise((resolve, reject) => {
    new PerformanceObserver((entryList) => {
      resolve(entryList.getEntries().filter(entry => entry.name === 'mousedown'));
    }).observe({ entryTypes: ['event'] });
  });
  window.addEventListener('load', e => {
    observerPromise.then((observedEntries) => {
      const onloadStart = performance.now();
      const bufferedEntries = performance.getEntriesByType('event');
      top.postMessage({
        "bufferedEntries" : bufferedEntries,
        "observedEntries": observedEntries,
        "clickTimeMin": clickTimeMin,
        "processingStartMin" : processingStartMin,
        "onloadStart" : onloadStart,
      }, '*');
    });
  });
</script>
</html>