File: shift-while-scrolled.html

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

body { height: 2000px; margin: 0; }
#shift { position: relative; width: 300px; height: 200px; background: blue; }

</style>
<div id="shift"></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);

  // Scroll down by 100px.
  document.scrollingElement.scrollTop = 100;
  assert_equals(watcher.score, 0);

  // Generate a layout shift.
  document.querySelector("#shift").style = "top: 60px";

  // Impact region: width * (height - scrollTop + moveDistance)
  const expectedScore = computeExpectedScore(
      300 * (200 - 100 + 60), 60);

  await watcher.promise;
  assert_equals(watcher.score, expectedScore);
}, 'Layout shift with non-zero scroll offset.');

</script>