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chromium/third_party/blink/web_tests/intersection-observer/resources/iframe-that-shares-observer.html 

<body>
<div id="root">
<div id="child"></div>
</div>
<script>
var o = new IntersectionObserver(function(){}, {root: root});
o.observe(child);
parent.move(root);
</script>
</body>