LayoutTests/fast/block/missing-fixed-positioned-content.html

<style>
html {
  will-change: transform;
}

.fixedPositioned {
  position: fixed;
}

.relativePositioned {
  position: relative;
}
</style>
<div id=container class=fixedPositioned>
  <div class=relativePositioned>
    <div class=fixedPositioned>
      <div id=mutateThis class=fixedPositioned></div>
    </div>
  </div>
</div>
<script>
window.testRunner?.dumpAsText();

document.body.offsetHeight;
container.style.willChange = "transform";
document.body.offsetHeight;
mutateThis.innerText = "PASS if this is visible";
</script>