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chromium/third_party/blink/web_tests/external/wpt/html/semantics/popovers/popover-open-display.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-open-display-ref.html">

<div popover>This is a popover</div>
<div class=topmost></div>

<style>
  .topmost {
    position:fixed;
    z-index: 999999;
    top:0;
    left:0;
    width:1000px;
    height:1000px;
    background:green;
    margin:0;
    padding:0;
  }
</style>

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