<!DOCTYPE html>
<div style="overflow-y: auto; height: 500px; width: 500px; background-color: pink; ">
  <div style="position: absolute; top: 0; width: 20px; height: 20px; background-color: green;"></div>
  <div style="height: 5000px; background-color: yellow;"></div>
</div>
<div style="background-color: lightgreen; position: fixed; top: 50px; left: 10px; height: 400px; width: 400px;">
  <div style="z-index: 0; position: relative">
    <div style="background-color: lightblue; position: fixed; top: 75px; left: 10px; height: 300px; width: 300px;"></div>
    <div style="background-color: salmon; height: 200px; position: absolute; width: 200px"></div>
  </div>
</div>
<script>
if (window.testRunner)
    internals.settings.setPreferCompositingToLCDTextEnabled(true);
</script>