chromium/third_party/blink/web_tests/external/wpt/scroll-animations/scroll-timelines/set-current-time-before-play.html

<html class="reftest-wait">
<title>Setting current time before play should not timeout</title>
<link rel="help" href="https://drafts.csswg.org/scroll-animations/">
<meta name="assert" content="Regression test to make sure the ready promise is correctly resolved">
<link rel="match" href="animation-ref.html">

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

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

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

  #scroller {
    overflow: auto;
    height: 100px;
    width: 100px;
    will-change: transform; /* force compositing */
  }

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

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

<script>
  async function runTest() {
    await waitForCompositorReady();

    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(
        { source: scroller, orientation: 'block' });
    const animation = new Animation(effect, timeline);
    animation.currentTime = CSS.percent(0);
    animation.play();

    animation.ready.then(() => {
      // Move the scroller to the halfway point.
      const maxScroll = scroller.scrollHeight - scroller.clientHeight;
      scroller.scrollTop = 0.5 * maxScroll;

      waitForAnimationFrames(2).then(_ => {
        takeScreenshot();
      });
    });
  }

  window.onload = runTest;
</script>