<!DOCTYPE HTML>
<body>
<script>
function syncXHR() {
let xhr = new XMLHttpRequest();
xhr.open("GET", window.location, false);
xhr.send(null);
}
addEventListener('load', evt => {
syncXHR();
document.open();
document.write(
'<body>' +
'<iframe src="about:blank"></iframe>' +
'<script>window.opener.postMessage("DONE", "*");</' + 'script>' +
'</body>');
document.close();
}, { once: true });
</script>
</body>