<style>
#container {
  float: left;
  font-family: monospace;
}
.parent {
  position: relative;
}
#mutate_this {
  position: absolute;
  background-color: green;
}
#remove_this {
  display: inline-block;
}
</style>
<div id=container>
  <div class=parent>
    <div id=mutate_this>
      <div id=remove_this></div>
    </div>
  </div>
</div>
<script>
document.body.offsetHeight;
container.style.position = "fixed";
remove_this.remove();

document.body.offsetHeight;
mutate_this.appendChild(document.createTextNode("PASS if this text is visible"));
</script>