css/css-contain/contain-layout-020.html

<!DOCTYPE html>
<html class="reftest-wait">
<meta charset=utf-8>
<title>CSS Containment Test: Removing layout containment and contained positioned elements</title>
<link rel="help" href="https://drafts.csswg.org/css-contain-1/#containment-layout">
<link rel="match" href="reference/contain-layout-020-ref.html">
<meta name=assert content="Removing layout containment relayouts contained positioned elements correctly.">

<style>
#container {
    width: 300px;
    height: 300px;
    contain: layout;
}
.box {
    width: 100px;
    height: 100px;
    background-color: green;
}
.fixed {
    position: fixed;
    top: 0;
    left: 0;
}
.abspos {
    position: absolute;
    top: 0;
    right: 0;
}
</style>

<div id="container">
  <div class="fixed box"></div>
  <div class="abspos box"></div>
</div>

<script>
  requestAnimationFrame(() => {
    container.style.contain = "none";
    document.documentElement.removeAttribute("class");
  });
</script>