fetch/api/resources/keepalive-iframe.html

<!doctype html>
<html>
<meta charset="utf-8">
<script>
const SEARCH_PARAMS = new URL(location.href).searchParams;
const ORIGIN = SEARCH_PARAMS.get('origin') || '';
const FRAME_ORIGIN = new URL(location.href).origin;
const TOKEN = SEARCH_PARAMS.get('token') || '';
const METHOD = SEARCH_PARAMS.get('method') || 'GET';
const SEND_ON_EVENT = SEARCH_PARAMS.get('sendOn') || 'load';
const MODE = SEARCH_PARAMS.get('mode') || 'cors';
const DISALLOW_CROSS_ORIGIN = SEARCH_PARAMS.get('disallowCrossOrigin') || '';
// CORS requests are allowed by this URL by default.
const url = `${ORIGIN}/fetch/api/resources/stash-put.py?key=${TOKEN}&value=on&mode=${MODE}`
    + `&frame_origin=${FRAME_ORIGIN}` + (DISALLOW_CROSS_ORIGIN ? `&disallow_cross_origin=1` : '');

addEventListener(SEND_ON_EVENT, () => {
  let p = fetch(url, {keepalive: true, method: METHOD, mode: MODE});
  window.parent.postMessage(TOKEN, '*');
});
</script>
</html>