<!DOCTYPE html>
<html class="test-wait">
<meta charset="utf-8">
<link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1822907">
<style>
  #inner {
    /* Extremely short so that we can just do a double-rAF and
     * expect that this transition will have completed: */
    transition: opacity 0.01s;
  }
</style>
<script>
  function setup() {
    // Flush style, in case the document hasn't been styled yet:
    let origOpacity = getComputedStyle(inner).opacity;

    // Trigger the (extremely short) transition, and proceed to next step
    // when it finishes.
    inner.addEventListener("transitionend", handleTransitionEnd);
    inner.style.opacity = "0.8";
  }
  function handleTransitionEnd(e) {
    // Hide & unhide the subtree that contained the transition:
    foo.style.contentVisibility = "hidden";
    document.documentElement.offsetHeight; // flush layout
    foo.style.contentVisibility = "";

    // Double-rAF to see if we crash when animations are resampled.
    requestAnimationFrame(() => { requestAnimationFrame(finish) });
  }
  function finish() {
    // If we get here, we've succeeded; hooray!
    document.documentElement.removeAttribute("class");
  }
</script>
<body onload="setup()">
  <div id="foo">
    Hello
    <div id="inner">Inner</div>
  </div>
</body>
</html>
