chromium/src/third_party/blink/web_tests/fast/scrolling/wheel-scrolling-over-custom-scrollbar.html

<!DOCTYPE>
<title>Wheel scrolling over custom scrollbar should scroll its scroller</title>
<script src="../../resources/testharness.js"></script>
<script src="../../resources/testharnessreport.js"></script>
<script src="../../resources/gesture-util.js"></script>
<style>
::-webkit-scrollbar {
  width: 50px;
  height: 50px;
  background-color: grey;
}
html,body {
  margin: 0;
}
.wheelDiv {
  width: 100px;
  height: 100px;
  overflow: auto;
  border: 1px solid black;
  margin: 20px;
}
.slow {
  clip: rect(0px, 1000px, 500px, 0px);
}
.fast {
  will-change: transform;
}
.space {
  height: 1000px;
}
</style>

<div class="wheelDiv slow">
  <div class="space">MAIN-THREAD SCROLLING</div>
</div>

<div class="wheelDiv fast">
  <div class="space">COMPOSITOR-THREAD SCROLLING</div>
</div>

<script>

function injectInput() {
  return smoothScroll(10, x, y, GestureSourceType.MOUSE_INPUT, "down");
}

window.onload = () => {
  const divSlow = document.querySelector('.slow');
  const divFast = document.querySelector('.fast');
  const rectSlow = divSlow.getBoundingClientRect();
  const rectFast = divFast.getBoundingClientRect();

  promise_test (async () => {
    await waitForCompositorCommit();

    const distance = 10;
    const source_type = GestureSourceType.MOUSE_INPUT; //Mouse == Wheel
    const direction = 'down';
    const use_precise_deltas = true;
    let x = rectSlow.right - 30;
    let y = rectSlow.top + 50;

    await smoothScroll(distance,
                       x, y,
                       source_type,
                       direction,
                       SPEED_INSTANT,
                       use_precise_deltas);

    assert_greater_than(divSlow.scrollTop, 0, "Main-thread scrolling div didn't scroll.");

    x = rectFast.right - 30;
    y = rectFast.top + 50;
    await smoothScroll(distance,
                       x, y,
                       source_type,
                       direction,
                       SPEED_INSTANT,
                       use_precise_deltas);
    assert_greater_than(divFast.scrollTop, 0, "Compositor-thread scrolling div didn't scroll.");
  }, "Test wheel scrolling over custom scrollbars");
}
</script>