chromium/third_party/blink/web_tests/fast/frames/transparent-scrollbar.html

<html>
<head>
<style>
::-webkit-scrollbar {
    height: 30px;
    width: 30px;
}

::-webkit-scrollbar-thumb {
    background-color: blue;
    height: 30px;
    width: 30px;
}

body {
  background-color: black;
}

</style>
<script src="../../resources/run-after-layout-and-paint.js"></script>
<script>
function test()
{
    runAfterLayoutAndPaint(function() {
        document.scrollingElement.scrollTop = 2000;
    }, true);
}
</script>
</head>
<body onload="test()">
<div style="height: 2000px; color: white;">PASS if blue scrollbar is at the bottom.</div>
</body>
</head>