third_party/blink/web_tests/scrollbars/overlay-scrollbars-within-overflow-scroll.html - chromium/src - Git at Google

 <!DOCTYPE html>

 <!--
 The scrollbar within the inner (blue) box should be visible and positioned at
 the top, even though the outer (green) box has been scrolled.
 -->
 <meta name=fuzzy content="maxDifference=0-1;totalPixels=0-500">
 <script>
 if (window.internals)
   internals.useMockOverlayScrollbars();
 </script>
 <style>
 html, body { margin: 0; padding: 0; }
 #outer {
     width: 400px;
     height: 300px;
     overflow-y: scroll;
     border: 1px solid green;
 }
 #inner {
     width: 100px;
     height: 100px;
     overflow-y: scroll;
     border: 1px solid blue;
 }
 .strut { height: 500px; }
 </style>

 <div id="outer">
     <div class="strut"></div>
     <div id="inner">
         <div class="strut"></div>
     </div>
 </div>

 <script>
 const outer = document.getElementById('outer');
 outer.scrollTop = outer.scrollHeight - outer.clientHeight;
 if (window.testRunner) {
     testRunner.dumpAsTextWithPixelResults();
     testRunner.setCustomTextOutput(internals.layerTreeAsText(document));
 }

 </script>
	<!DOCTYPE html>

	<!--
	The scrollbar within the inner (blue) box should be visible and positioned at
	the top, even though the outer (green) box has been scrolled.
	-->
	<meta name=fuzzy content="maxDifference=0-1;totalPixels=0-500">
	<script>
	if (window.internals)
	internals.useMockOverlayScrollbars();
	</script>
	<style>
	html, body { margin: 0; padding: 0; }
	#outer {
	width: 400px;
	height: 300px;
	overflow-y: scroll;
	border: 1px solid green;
	}
	#inner {
	width: 100px;
	height: 100px;
	overflow-y: scroll;
	border: 1px solid blue;
	}
	.strut { height: 500px; }
	</style>

	<div id="outer">
	<div class="strut"></div>
	<div id="inner">
	<div class="strut"></div>
	</div>
	</div>

	<script>
	const outer = document.getElementById('outer');
	outer.scrollTop = outer.scrollHeight - outer.clientHeight;
	if (window.testRunner) {
	testRunner.dumpAsTextWithPixelResults();
	testRunner.setCustomTextOutput(internals.layerTreeAsText(document));
	}

	</script>