layout-viewport-has-overlay-scrollbar.html

<!DOCTYPE HTML>
<script>
if (window.internals)
    internals.settings.setViewportEnabled(true);
</script>
<script src="../resources/testharness.js"></script>
<script src="../resources/testharnessreport.js"></script>
<div style="width:40px;height:3000px;background:green"></div>
<script>
  promise_test(async t => {
    await new Promise(resolve => { requestAnimationFrame(resolve); });

    assert_equals(innerWidth, document.scrollingElement.clientWidth);
  }, "Scrollbar becomes overlay when viewport setting is enabled.");
</script>

chromium/third_party/blink/web_tests/scrollbars/layout-viewport-has-overlay-scrollbar.html