LayoutTests/fast/dynamic/out-of-flow-with-inflow-sibling-insert.html

<style>
.in_flow {
  width: 100px;
  height: 100px;
  background-color: green;
}

.out_of_flow {
  z-index: -10;
  width: 100px;
  height: 100px;
  background-color: red;
  position: absolute;
}

.flex {
  display: flex;
}
</style>
<div class=flex>
  <div class=flex id=container style="position: relative"></div>
</div>
<script>
document.body.offsetHeight;
let out_of_flow = document.createElement("div");
out_of_flow.className = "out_of_flow";
container.appendChild(out_of_flow);

let in_flow = document.createElement("div");
in_flow.className = "in_flow";
container.appendChild(in_flow);
</script>