layout-instability/transform-counter-layout-shift.html

<!DOCTYPE html>
<title>Layout Instability: no layout shift if transform change counters location change</title>
<link rel="help" href="https://wicg.github.io/layout-instability/" />
<style>
body { margin: 0; }
#transformed { position: relative; transform: translateX(20px); width: 100px; height: 100px; background: blue; }
#child { width: 400px; height: 400px; }
</style>
<div id="transformed">
  <div id="child"></div>
</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);

  // Modify the transform and the location at the same time, and the values
  // cancel each other visually, for which no shift should be reported.
  transformed.style.transform = 'translateY(100px)';
  transformed.style.top = '-100px';
  transformed.style.left = '20px';
  // Change size of child, for which no shift should be reported, either.
  child.style.width = '300px';

  await waitForAnimationFrames(2);
  // No shift should be reported.
  assert_equals(watcher.score, 0);
}, 'no layout shift if transform change counters location change');

</script>