mozilla-release/testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline-and-overflow-hidden.https.html

<html class="reftest-wait">
<title>Scroll timeline with WorkletAnimation using a scroller with overflow hidden</title>
<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
<meta name="assert" content="Worklet animation correctly updates values when using a overflow: hidden on the scroller being used as the source for the ScrollTimeline">
<link rel="match" href="worklet-animation-with-scroll-timeline-and-overflow-hidden-ref.html">

<script src="/web-animations/testcommon.js"></script>
<script src="/common/reftest-wait.js"></script>
<script src="common.js"></script>

<style>
  #box {
    width: 100px;
    height: 100px;
    background-color: green;
  }

  #covered {
    width: 100px;
    height: 100px;
    background-color: red;
  }

  #scroller {
    overflow: hidden;
    height: 100px;
    width: 100px;
  }

  #contents {
    height: 1000px;
    width: 100%;
  }
</style>

<div id="box"></div>
<div id="covered"></div>
<div id="scroller">
  <div id="contents"></div>
</div>

<script>
  registerPassthroughAnimator().then(_ => {
    const box = document.getElementById('box');
    const effect = new KeyframeEffect(box,
      [
        {transform: 'translateY(0)', opacity: 1},
        {transform: 'translateY(200px)', opacity: 0}
      ], {
        duration: 1000,
      }
    );

    const scroller = document.getElementById('scroller');
    const timeline = new ScrollTimeline({ scrollSource: scroller, timeRange: 1000, orientation: 'block' });
    const animation = new WorkletAnimation('passthrough', effect, timeline);
    animation.play();

    // Move the scroller to the halfway point.
    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
    scroller.scrollTop = 0.5 * maxScroll;
    waitForAnimationFrameWithCondition(_ => {
      return getComputedStyle(box).transform != 'matrix(1, 0, 0, 1, 0, 0)';
    }).then(_ => {
      takeScreenshot();
    });
  });
</script>