<!doctype html>
<html>
<head>
<script type="text/javascript">
function noop() {}
function crash() {
  let div = document.querySelector("div");
  let ifr = document.querySelector("iframe");

  // We need a reference to the iframe's window from *before* it gets detached.
  let win = ifr.contentWindow;

  div.appendChild(ifr);

  win.addEventListener("beforeunload", noop);
  win.removeEventListener("beforeunload", noop);
}
</script>
</head>
<body onload="crash()">
  <div></div>
  <iframe></iframe>
</body>
</html>
