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

<!DOCTYPE html>
<title>Layout Instability: no shift for adding/removing position:fixed</title>
<link rel="help" href="https://wicg.github.io/layout-instability/" />
<div id="target" style="width: 100px; height: 100px; background: green;
                        position: absolute; top: 200px"></div>
<div style="height: 2000px"></div>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="resources/util.js"></script>
<script>

promise_test(async () => {
  const watcher = new ScoreWatcher;

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

  window.scrollTo(0, 1000);
  target.style.position = 'fixed';
  target.style.top = 0;

  await waitForAnimationFrames(3);
  assert_equals(watcher.score, 0);

  window.scrollTo(0, 100);
  target.style.position = 'absolute';
  target.style.top = '200px';

  await waitForAnimationFrames(3);
  assert_equals(watcher.score, 0);
}, "No shift for adding/removing position:fixed.");

</script>