chromium/third_party/blink/web_tests/external/wpt/html/browsers/browsing-the-web/unloading-documents/prompt/001-1.html

<script>
addEventListener("beforeunload",
function() {
   parent.events.push("beforeunload");
}, false);
parent.events.push("before src change");

location.href = "001-2.html&pipe=trickle(d2)";
parent.events.push("after src change");
</script>