expand-above-viewport.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/expand-above-viewport.html - WPT Dashboard Interop Dashboard

<!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>