css/css-transforms/dynamic-fixed-pos-cb-change.html

<!doctype html>
<html class="reftest-wait">
<title>CSS Test: Checks that dynamic changes to whether we're a fixed-pos containing block are handled correctly</title>
<meta charset="utf-8">
<link rel="help" href="https://drafts.csswg.org/css-transforms-1/#containing-block-for-all-descendants">
<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1498873">
<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
<link rel="author" title="Mozilla" href="https://mozilla.org">
<link rel="match" href="dynamic-fixed-pos-cb-change-ref.html">
<style>
html,
body {
  height: 100%;
  overflow: hidden;
  margin: 0;
  padding: 0;
  background-color: #222;
}

body {
  display: flex;
}

#fixed {
  position: fixed;
  width: 100%;
  height: 100%;
  top: 0;
  left: 0;
  z-index: 1;
  background: black;
}

#popup {
  position: absolute;
  width: 200px;
  height: 200px;
  left: 0;
  top: 0;
  z-index: 2;
  background: red;
  transform: scale( 1.1111 );
}

body, #popup {
  transform-origin: 50% 50%;
}
</style>
<div id="popup">
  <!-- Seeing this box change color should not move me or change my size. -->
</div>
<div id="fixed"></div>
<script>
onload = function() {
  requestAnimationFrame(() => {
    document.body.style.transform = "scale(0.9)";
    requestAnimationFrame(() => {
      popup.style.backgroundColor = "green";
      requestAnimationFrame(() => {
        document.documentElement.className = "";
      });
    });
  });
}
</script>