LayoutTests/fast/dynamic/gap-style-change-should-initiate-layout.html

<style>
#grid {
  width: 200px;
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  grid-gap: 0px;
  overflow: hidden;
}

#flex {
  width: 200px;
  display: flex;
  flex-direction: row;
}

.item {
  width: 100px;
  height: 100px;
  background-color: green;
}
</style>
<div id=grid><div class=item></div><div class=item></div></div>
<div id=flex><div class=item></div><div class=item></div></div>
<script>
  document.body.offsetHeight;
  grid.style.gridGap = "200px";
  flex.style.gap = "200px";
</script>