fuchsia_web/webengine/test/data/report_close_events.html

<script>
  window.eventLogPort = null;
  window.shouldHangOnEvent = false;

  window.addEventListener('message', function(event) {
    console.assert(event.ports && event.ports.length == 1);
    window.eventLogPort = event.ports[0]
    window.shouldHangOnEvent = event.data == 'hang_on_event';
  });

  let event_list = [
    'beforeunload',
    'pagehide',
    'unload',
  ];

  for (const event of event_list) {
    window.addEventListener(event, event => {
      window.eventLogPort.postMessage(`window.${event.type}`);
      if (window.shouldHangOnEvent) {
        while (true) {}
      }
    });
  }
</script>