chromium/third_party/blink/web_tests/wpt_internal/css/css-overflow/scroll-buttons-selection.html

<!doctype html>
<meta charset="utf-8">
<title>CSS Test: ::scroll-marker click with active selection</title>
<link rel="help" href="https://drafts.csswg.org/css-overflow-5/#scroll-marker-pseudo">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="/resources/testdriver.js"></script>
<script src="/resources/testdriver-actions.js"></script>
<script src="/resources/testdriver-vendor.js"></script>
<style>
    * {
      margin: 0;
      padding: 0;
    }

    #scroller {
        width: 600px;
        height: 300px;
        overflow: scroll;
        scroll-marker-group: before;
    }

    #scroller div {
        width: 500px;
        height: 300px;
        margin-bottom: 20px;
        background: green;
    }

    #scroller::scroll-marker-group {
        border: 3px solid black;
        padding: 5px;
        display: block;
    }

    #scroller div::scroll-marker {
        content: "";
        width: 50px;
        height: 50px;
        background-color: blue;
        display: inline-block;
    }

    #scroller #first::scroll-marker {
        background-color: purple;
        margin-right: 4px;
    }
</style>
<div id="scroller" tabindex=0>
    <div id="first">Hello</div>
    <div>World</div>
</div>
<script>
  selection = document.getSelection();
  selection.setBaseAndExtent(first, 0, first, 1);

  promise_test(async t => {
    actions_promise = new test_driver.Actions()
    .pointerMove(20, 20)
    .pointerDown()
    .pointerUp()
    .send();
    await actions_promise;
    assert_equals(selection.anchorNode, null);
  });
</script>