<div id="parent">
  <div id="fixed">
  </div>
</div>
<!-- 'fixed' should overflow the bounds of 'parent', because 'parent' is not
the containing block for 'fixed' -->
<style>
#parent {
  position: absolute;
  z-index: 0;
  overflow: auto;
  background: lightgray;
  width: 150px;
  height: 150px;
  backface-visibility: hidden;
}

#fixed {
  position: fixed;
  top: 30px;
  left: 30px;
  height: 200px;
  width: 200px;
  background: lightblue;
}
</style>