<style>
div {
  background-color: green;
  height: 20px;
  width: 20px;
  font-size: 12px;
  color: transparent;
  font-family: Monospace;
}

.flex {
  display: flex;
}

.out_of_flow {
  display: none;
  position: absolute;
  margin-top: 20px;
}
</style>
<div class=flex>
  PASS if no
  <div id=show_this class=out_of_flow>gap</div>
</div>
<script>
document.body.offsetHeight;
show_this.style.display = "block";
</script>
