third_party/blink/web_tests/shadow-dom/layout-1.html - chromium/src - Git at Google

 <!DOCTYPE html>
 <div id='host'>Child</div>
 <script>
 const host = document.querySelector('#host');
 host.offsetLeft;  // Force layout
 host.attachShadow({ 'mode': 'open' });  // Child should be detached
 </script>
	<!DOCTYPE html>
	<div id='host'>Child</div>
	<script>
	const host = document.querySelector('#host');
	host.offsetLeft; // Force layout
	host.attachShadow({ 'mode': 'open' }); // Child should be detached
	</script>