<!DOCTYPE html>
<script>
if (window.internals)
  internals.useMockOverlayScrollbars();
</script>
<div id="container" style="width: 200px; height: 200px; overflow: scroll; position: relative">
  <div id="content" style="position: absolute; z-index: 10; background: green; width: 210px; height: 150px"></div>
  <div style="height: 210px"></div>
</div>
<div style="position: absolute; top: 80px; left: 80px; z-index: 5; background: blue; width: 200px; height: 200px"></div>
<div style="position: absolute; top: 120px; left: 120px; z-index: 20; background: yellow; width: 200px; height: 200px"></div>