css/css-flexbox/position-absolute-004.html

<!DOCTYPE html>
<title>CSS Test: Absolutely positioned children of flexboxes</title>
<link rel="author" title="Google Inc." href="http://www.google.com/">
<link rel="help" href="https://drafts.csswg.org/css-flexbox/#abspos-items">
<meta name="flags" content="dom">
<meta name="assert" content="Checks that we correctly handle when a flex item
becomes absolutely positioned">
<style>
#flex {
  display: flex;
  position: relative;
  background: red;
  width: 500px;
  height: 200px;
}

#item {
  background: green;
  left: 0;
  right: 0;
  top: 0;
  bottom: 0;
}
</style>

<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="/resources/check-layout-th.js"></script>

<script>
function update() {
  var item = document.getElementById("item");
  item.offsetHeight;
  item.style.position = "absolute";
  item.offsetHeight;
  checkLayout("#flex");
}
</script>

<body onload="update();">

<div id="flex">
  <div id="item" data-expected-width="500"></div>
</div>