<!DOCTYPE html>
<meta charset="utf-8">
<title>CSS transforms: Creating containing block for fixed positioned elements</title>
<link rel="author" title="L. David Baron" href="https://dbaron.org/">
<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
<link rel="help" href="https://drafts.csswg.org/css-transforms-1/#transform-rendering">
<link rel="help" href="https://drafts.csswg.org/css-transforms-1/#transform-property">
<link rel="match" href="containing-block-dynamic-1-ref.html">
<meta name="assert" content="For elements whose layout is governed by the CSS box model, any value other than none for the transform results in the creation of both a stacking context and a containing block. The object acts as a containing block for fixed positioned descendants.">
<meta name="assert" content="The object acts as a containing block for fixed positioned descendants."
<meta name="flags" content="dom">
<style>
  html, body { margin: 0; padding: 0 }
  #changetransform {
    position: absolute;
    top: 100px;
    left: 100px;
  }

  #abscovered {
    position: absolute;
    top: 50px;
    left: 50px;
    background: red;
    height: 100px;
    width: 100px;
  }

  #fixedmoves {
    position: fixed;
    top: 50px;
    left: 50px;
    background: green;
    height: 100px;
    width: 100px;
  }
</style>
<body>
  <div id="changetransform">
    <div id="abscovered"></div>
    <div id="fixedmoves"></div>
  </div>
  <script>
    var changetransform = document.getElementById("changetransform");
    var fixedmoves = document.getElementById("fixedmoves");
    var causeFlush = fixedmoves.offsetTop;
    changetransform.style.transform = "translateX(0px)";
  </script>
</body>