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chromium/third_party/blink/web_tests/virtual/overlay-scrollbar/overlay-scrollbar-smoke-test.html 

<head>
  <title>Overlay Scrollbars test.</title>
</head>
<body>
<div style="width: 200px; height: 200px; border: 1px solid; background-color:teal; overflow:scroll;">
  <div style="width: 800px; height: 800px;"></div>
</div>
<div style="width: 1000px; height: 1000px;"></div>
</body>