chromium/third_party/blink/web_tests/fast/scrolling/fixed-position-behind-scrollbar.html

<!DOCTYPE html>
<script src='../../resources/testharness.js'></script>
<script src='../../resources/testharnessreport.js'></script>
<script src='../../resources/gesture-util.js'></script>
<style>
  #f {
    position: fixed;
    background-color: #def;
    width: 600px;
    height: 400px;
    right: -20px;
    bottom: -20px;
  }

  body {
    width: 1000px;
    height: 1000px;
  }
</style>

<div id='f'></div>

<script>
  if (window.internals)
    internals.settings.setScrollAnimatorEnabled(false);

  window.onload = async () => {
    // Give rAFs ensure layer information has gone from Blink to CC's active tree.
    await waitForCompositorCommit();

    promise_test(async () => {
      await mouseClickOn(790, 500);
      // Will throw exception if window.scrollY not greater than 0.
      await waitFor(() => { return window.scrollY > 0; });

      let rect = document.querySelector('#f').getBoundingClientRect();
      assert_equals(rect.left, 205);
      assert_equals(rect.top, 205);
    }, 'Tests that the scrollbar can be clicked even when it clips a position:fixed' +
    'element, and that the scrollbars are excluded from the viewport for determining' +
    'the element\'s position.');
  }
</script>