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chromium/third_party/blink/web_tests/external/wpt/layout-instability/fixed-position-move.html 

<!DOCTYPE html>
<title>Layout Instability: movement of fixed position</title>
<link rel="help" href="https://wicg.github.io/layout-instability/" />
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="resources/util.js"></script>
<div id="target" style="position: fixed; top: 100px; width: 300px; height: 200px; background: yellow">
</div>
<script>
promise_test(async () => {
  const watcher = new ScoreWatcher;

  // Wait for the initial render to complete.
  await waitForAnimationFrames(2);

  // Modify the position of the div.
  target.style.top = '200px';

  const expectedScore = computeExpectedScore(300 * (200 + 100), 100);

  // Observer fires after the frame is painted.
  assert_equals(watcher.score, 0);
  await watcher.promise;
  assert_equals(watcher.score, expectedScore);
}, 'Movement of fixed position');

</script>