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chromium/third_party/blink/web_tests/external/wpt/layout-instability/transform.html 

<!DOCTYPE html>
<title>Layout Instability: shift inside a transformed container</title>
<link rel="help" href="https://wicg.github.io/layout-instability/" />
<style>

body { margin: 0; }
#container { transform: translateX(-300px) translateY(-40px); }
#shifter { position: relative; width: 600px; height: 140px; background: blue; }

</style>
<div id="container">
  <div id="shifter"></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 position of the div.
  document.querySelector("#shifter").style = "top: 60px";

  // The shifter has size 600 x 140, but the container's transform
  // reduces its viewport overlap.
  const expectedScore = computeExpectedScore(
      (600 - 300) * (140 - 40 + 60), 60);

  await watcher.promise;
  assert_equals(watcher.score, expectedScore);
}, 'Transformed container.');

</script>