fetch/api/resources/keepalive-window.html

<!doctype html>
<html>
<meta charset="utf-8">
<script src="/common/utils.js"></script>
<script src="/common/get-host-info.sub.js"></script>
<script>
const TOKEN = token();
const {
  HTTP_NOTSAMESITE_ORIGIN,
  HTTP_REMOTE_ORIGIN,
  HTTP_REMOTE_ORIGIN_WITH_DIFFERENT_PORT
} = get_host_info();
const REDIRECT_DESTINATION =
  `${HTTP_REMOTE_ORIGIN_WITH_DIFFERENT_PORT}/fetch/api/resources/stash-put.py` +
  `?key=${TOKEN}&value=on`;
const URL =
    `${HTTP_REMOTE_ORIGIN}/fetch/api/resources/redirect.py?` +
    `delay=500&` +
    `allow_headers=foo&` +
    `location=${encodeURIComponent(REDIRECT_DESTINATION)}`;

addEventListener('load', () => {
  const iframe = document.createElement('iframe');
  document.body.appendChild(iframe);
  iframe.contentWindow.addEventListener('unload', () => {
    iframe.contentWindow.fetch(URL, {keepalive: true, headers: {foo: 'bar'}});
  });

  window.opener.postMessage(TOKEN, '*');
  // Do NOT remove `iframe` here. We want to check the case where the nested
  // frame is implicitly closed by window closure.
});
</script>
</html>