css/css-scrollbars/scrollbar-width-013.html

<!doctype html>
<meta charset="utf-8">
<title>CSS Scrollbars: scrollbar-width thin on the root overridess ::-webkit-scrollbar</title>
<link rel="author" title="Luke Warlow" href="mailto:luke@warlow.dev" />
<link rel="help" href="https://drafts.csswg.org/css-scrollbars-1/#width-compat" />
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="/css/support/parsing-testcommon.js"></script>
<style>
  :root {
    scrollbar-width: thin;
  }

  ::-webkit-scrollbar {
    display: none;
  }

  :root,
  body {
    margin: 0;
    padding: 0;
  }

  #content {
    height: 10vh;
    width: 100%;
    background: lightsalmon;
  }

  #expander {
    /* force vertical scroll */
    height: 200vh;
    width: 300px;
    background: gray;
  }
</style>

<body>

  <div id="content"></div>

  <div id="expander"></div>

  <script>
    test(function () {
      let root = document.documentElement;
      let body = document.body;
      let content = document.getElementById('content');

      assert_less_than(root.offsetWidth, window.innerWidth, "viewport has a scrollbar");
      assert_equals(body.offsetWidth, root.offsetWidth, "body matches root");
      assert_equals(content.offsetWidth, body.offsetWidth, "content matches body");
    }, "scrollbar-width thin on the root overridess ::-webkit-scrollbar");
  </script>
</body>