<!DOCTYPE html>
<html>
<head>
    <style>
        .container {
            position: relative;
            z-index: 0;
            width: 200px;
            height: 200px;
            border: 1px solid green;
        }

        .fixed {
            position: fixed;
            left: 0;
            width: 100%;
            height: 100px;
            background-color: rgba(0, 0, 0, 0.2);
        }
        .top {
            top: 0;
        }
        .bottom {
            bottom: 0;
        }
    </style>
    <script>
        if (window.testRunner)
            testRunner.dumpAsText();
        window.addEventListener('load', () => {
            if (window.internals)
                document.getElementById('scrollingtree').textContent = internals.scrollingStateTreeAsText();
            
        }, false);
    </script>
</head>
<body>
    <div class="container">
        <div class="top fixed">
            Top
        </div>
    </div>

    <div class="bottom fixed">
        Bottom
    </div>
<pre id="scrollingtree"></pre>
</body>
</html>