layout-instability/sub-frame.html

<!DOCTYPE html>
<html>
<body>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="resources/test-adapter.js"></script>
<script src="resources/util.js"></script>
<style>
  #j {
    position: relative;
    width: 300px;
    height: 100px;
    background-color: purple;
  }
</style>
<div id="j"></div>
<script>
function shiftFrame() {
  document.getElementById('j').style.top = '60px';
}
function unshiftFrame() {
  document.getElementById('j').style.top = '';
}

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

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

  const expectedScore = computeExpectedScore(300 * (100 + 60), 60);

  cls_expect(watcher, {score: 0});
  await watcher.promise;
  cls_expect(watcher, {score: expectedScore});

  unshiftFrame();
  await watcher.promise;
  cls_expect(watcher, {score: expectedScore * 2});

  window.parent.postMessage({
    type: 'layout shift score',
    score: watcher.score,
    expectedScore: expectedScore * 2,
  }, '*');
}, 'We will see two layout shift with the same score in the subframe.');
</script>
</body>
</html>