LayoutTests/fast/repaint/no-backdrop-repaint.html

<style>
[popover] {
  margin: 50px;
  border: none;
}
:popover-open {
  width: 100px;
  height: 100px;
  position: absolute;
  background-color: red;
}

.container {
  width: 100px;
  height: 100px;
  position: absolute;
  left: 200px;
  top: 200px;
}
input {
  display: none;
}
</style>
<!-- PASS if no red -->
<input type="button" popovertarget="mypopover" popovertargetaction="show">
<div class=container><div id="mypopover" popover=""></div></div>
<pre id=repaints></pre>
<script src="../../resources/ui-helper.js"></script>
<script>
const popover = document.querySelector('[popover]')
popover.showPopover();

if (window.testRunner) {
  testRunner.dumpAsText();
  testRunner.waitUntilDone();
}

async function runTest() {
  await UIHelper.renderingUpdate();

  if (window.internals)
    internals.startTrackingRepaints();

  popover.hidePopover();

  await UIHelper.renderingUpdate();

  if (window.internals) {
    repaints.innerHTML = window.internals.repaintRectsAsText();
    internals.stopTrackingRepaints();
  }
  
  if (window.testRunner)
    testRunner.notifyDone();
}

window.addEventListener('load', runTest);
</script>