mozilla-release/testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline-and-display-none.https.html

<html class="reftest-wait">
<title>Scroll timeline with WorkletAnimation and transition from display:none to display:block</title>
<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
<meta name="assert" content="Scroll timeline should properly handle going from display:none to display:block">
<link rel="match" href="worklet-animation-with-scroll-timeline-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;
  }

  /* Hide scrollbars to avoid unnecessary visual issues related to them */
  #scroller::-webkit-scrollbar {
    display: none;
  }

  #scroller {
    scrollbar-width: none;
    overflow: auto;
    height: 100px;
    width: 100px;
    will-change: transform; /* force compositing */
  }

  .removed {
    display: none;
  }

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

    // Ensure that the WorkletAnimation will have been started on the compositor.
    waitForAsyncAnimationFrames(1).then(_ => {
      // Now return the scroller to the world, which will cause it to be composited
      // and the animation should update on the compositor side.
      scroller.classList.remove('removed');
      const maxScroll = scroller.scrollHeight - scroller.clientHeight;
      scroller.scrollTop = 0.5 * maxScroll;

      waitForAsyncAnimationFrames(1).then(_ => {
        takeScreenshot();
      });
    });
  });
</script>