transform-counter-layout-shift.html

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This test has a WPT meta file that expects 1 subtest issues.
This WPT test may be referenced by the following Test IDs:
- /layout-instability/transform-counter-layout-shift.html - WPT Dashboard Interop Dashboard

<!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>