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chromium/third_party/blink/web_tests/scrollbars/overlay-scrollbar-over-child-layer-nested-expected.html 

<!DOCTYPE html>
<script>
if (window.internals)
  internals.useMockOverlayScrollbars();
</script>
<div style="width: 200px; height: 200px; overflow: scroll">
  <div style="width: 210px; height: 80px; overflow: scroll">
    <div style="background: green; width: 300px; height: 200px"></div>
  </div>
  <div style="height: 210px"></div>
</div>