css/css-scroll-anchoring/after-scrollable-range-shrinkage-002.html

<!DOCTYPE html>
<html>
<meta name="viewport" content="width=device-width">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<link rel="author" title="Mozilla" href="https://mozilla.org">
<link rel="help" href="https://drafts.csswg.org/css-scroll-anchoring/">
<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1779404">
<div style="height: 500vh;"></div>
<div id="anchor" style="height: 10px; background-color: blue;"></div>
<div style="height: 200vh;"></div>
<script>
promise_test(async t => {
  assert_equals(window.scrollY, 0);

  let anchorRect = anchor.getBoundingClientRect();
  // Scroll to the anchor node.
  window.scrollBy(0, anchorRect.y);
  assert_equals(window.scrollY, anchorRect.y);

  await new Promise(resolve => requestAnimationFrame(resolve));
  await new Promise(resolve => t.step_timeout(resolve, 0));

  // Shrink the first element so that scroll anchoring happens.
  document.querySelectorAll("div")[0].style.height = "100vh";

  await new Promise(resolve => requestAnimationFrame(resolve));
  await new Promise(resolve => requestAnimationFrame(resolve));

  const scrollAnchorPosition = window.scrollY;

  // Scroll back to (0, 0) to calculate the expected scroll anchor element
  // position.
  window.scrollTo(0, 0);
  anchorRect = anchor.getBoundingClientRect();
  assert_equals(scrollAnchorPosition, anchorRect.y);
}, "Scroll anchoring properly works after scrollable range shrinkage");
</script>
</html>