<!DOCTYPE html>
<title>Layout Instability: layout shift when content expanded above the viewport</title>
<link rel="help" href="https://wicg.github.io/layout-instability/" />
<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>
body {
  margin: 0;
  /* To avoid browser automatic scroll offset adjustment for the expansion. */
  overflow-anchor: none;
}
</style>
<div id="expander" style="height: 50vh"></div>
<div id="shifted" style="width: 300px; height: 300vh; background: blue"></div>
<script>

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

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

  const viewHeight = document.documentElement.clientHeight;
  window.scrollTo(0, viewHeight);

  await waitForAnimationFrames(2);

  // Expander expands to push |shifted| down.
  expander.style.height = '150vh';

  const expectedScore1 = computeExpectedScore(300 * viewHeight, viewHeight);

  // Observer fires after the frame is painted.
  cls_expect(watcher, {score: 0});
  await watcher.promise;
  cls_expect(watcher, {score: expectedScore1});

  // Expander expands to push |shifted| out of viewport.
  expander.style.height = '200vh';

  const expectedScore2 = expectedScore1 +
                         computeExpectedScore(0.5 * 300 * viewHeight, 0.5 * viewHeight);
  await watcher.promise;
  cls_expect(watcher, {score: expectedScore2});
}, "Layout shift when content expanded above the viewport");

</script>
