<style>
body { height: 10000px; }
.fixed { position:fixed; background-color: green; width: 200px; height:200px; left: 100px;}
.clipped-fixed { position: fixed; top: 50px; left: 40px; width: 200px; height:20px; clip:rect(0,0,0,0)}
</style>
<div class=fixed><div class=clipped-fixed>text</div></div>
<pre id=results></pre>
<script>
if (window.internals) {
    testRunner.dumpAsText();

    const result1 = internals.layerTreeAsText(document);
    document.querySelector('.fixed').style.color = "blue";
    const result2 = internals.layerTreeAsText(document);

    const pass = result1 == result2;
    results.textContent = pass ? "PASS" : "FAIL";
    if (!pass)
        results.textContent +=  "\n" + result1 + "\n" + result2;
}
</script>