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chromium/third_party/blink/web_tests/external/wpt/html/semantics/popovers/popover-stacking-context.html 

<!DOCTYPE html>
<meta charset="utf-8">
<link rel="author" href="mailto:[email protected]">
<link rel=help href="https://open-ui.org/components/popover.research.explainer">
<link rel=help href="https://html.spec.whatwg.org/multipage/popover.html">
<link rel=match href="popover-stacking-context-ref.html">

<div popover>
  Inside popover
  <div class=z style="z-index: 2; background:lightgreen">z-index 2
    <div class=z style="z-index: 3; background:lightblue; left: 20px;">z-index 3</div>
    <div class=z style="z-index: 1; background:pink; top:-20px; left: 10px;">z-index 1</div>
  </div>
  <div class=z style="background:green; top:-100px; left: 250px; width: 100px;">Outside</div>
  Bottom of popover
</div>

<style>
  [popover] {
    width: 200px;
    height: 230px;
    border: 1px solid red;
    top:50px;
    left:50px;
  }
  .z {
    position: relative;
    border: 1px solid black;
    padding: 1em;
  }
</style>

<script>
  document.querySelector('[popover]').showPopover();
</script>