css/css-position/containing-block-change-scrollframe.html

<!doctype html>
<meta charset="utf-8">
<title>Tests that after making a scrolling box a containing block, dynamically-inserted abspos boxes still get positioned correctly</title>
<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1735533">
<link rel="help" href="https://drafts.csswg.org/css-position/#abspos-insets">
<link rel="author" href="mailto:emilio@crisal.io" title="Emilio Cobos Álvarez">
<link rel="author" href="https://mozilla.org" title="Mozilla">
<link rel="match" href="containing-block-change-scrollframe-ref.html">
<style>
  #container {
    width: 400px;
    height: 400px;
    margin: 100px;
    background-color: purple;
    overflow: hidden;
  }
  #spacer {
    height: 700px;
  }
  #bottom {
    height: 100px;
    background-color: green;
  }
  #abspos {
    position: absolute;
    width: 100px;
    height: 100px;
    background-color: green;
    top: 400px;
    left: 0;
  }
</style>
<div id="container">
  <div id="spacer"></div>
  <div id="bottom"></div>
</div>
<script>
  let container = document.getElementById("container");
  container.scrollTop = 400;
  container.getBoundingClientRect();
  container.style.position = "relative";
  container.getBoundingClientRect();
  let abspos = document.createElement("div");
  abspos.id = "abspos";
  container.appendChild(abspos);
</script>