custom-scrollbar-width-change-on-viewport.html

<!doctype html>
<title>Changing width of scrollbar propagated from body to viewport</title>
<style>
  body { overflow: scroll }
  body::-webkit-scrollbar {
    background: red;
    width: 200px;
    height: 200px;
  }
  .narrow::-webkit-scrollbar {
    width: 0px;
    height: 0px;
  }
</style>
<body>
<script>
  document.body.offsetTop;
  document.body.className = "narrow";
</script>

chromium/third_party/blink/web_tests/scrollbars/custom-scrollbar-width-change-on-viewport.html