css/css-position/hypothetical-dynamic-change-001.html

<!doctype html>
<title>CSS test: movement of fixed-position ancestor correctly moves fixed-position descendant depending on the static position</title>
<link rel="help" href="https://drafts.csswg.org/css-position/#size-and-position-details">
<link rel="match" href="hypothetical-dynamic-change-001-ref.html">
<link rel="author" href="mailto:emilio@crisal.io" title="Emilio Cobos Álvarez">
<link rel="author" href="https://mozilla.org" title="Mozilla">
<style>
  .ancestor, .child {
    position: fixed;
    width: 100px;
    height: 100px;
    background-color: green;
    /* NOTE: child remains auto-positioned */
  }
  .ancestor {
    left: 0;
    top: 0;
    background-color: red;
  }
</style>
<div class="ancestor">
  <div class="child"></div>
</div>
<script>
onload = function() {
  let ancestor = document.querySelector(".ancestor");
  window.unused = ancestor.getBoundingClientRect();
  ancestor.style.left = "100px";
}
</script>