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<!DOCTYPE HTML>
<meta charset=utf-8>
<meta name="viewport" content="width=device-width">
<title>Long Task Frame</title>
<body>
<h1>Long Task plus PostMessage</h1>
<script>
const begin = window.performance.now();
while (window.performance.now() < begin + 60);
window.opener.postMessage('Finished.', '*');
</script>
</body>