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chromium/third_party/blink/web_tests/external/wpt/dom/events/resources/event-global-is-still-set-when-coercing-beforeunload-result-frame.html 

<!doctype html>
<meta charset="utf-8">
<body>
<script>
window.onbeforeunload = () => ({ toString: () => { parent.currentEventInToString = window.event; } });
</script>