css/css-contain/contain-layout-021.html

<!DOCTYPE html>
<meta charset="utf-8">
<title>CSS Containment Test: Layout containment fixed positioned descendants</title>
<link rel="help" href="https://drafts.csswg.org/css-contain-1/#containment-layout">
<link rel="match" href="reference/contain-layout-021-ref.html">
<meta name=assert content="Layout containment makes an element to act as containing block for fixed positioned descendants.">
<style>
html {
  contain: layout;
}
#fixed {
  position: fixed;
  background: green;
  width: 100px;
  height: 100px;
}
body {
  height: 3000px;
  margin: 0px;
}
#spacer {
  height: 200px;
}
</style>
<script>
function runTest() {
  document.documentElement.scrollTop += 200;
}
</script>
<body onload="runTest()">
<div id="spacer"></div>
<p>Test passes if there is a filled green square and <strong>no red</strong>.</p>
<div id="fixed"></div>
</body>