chromium/third_party/blink/web_tests/compositing/iframes/resizer.html

<!DOCTYPE html>
<style>
.container {
     will-change: transform;
     resize: both;
}
</style>
<script>
function test() {
    if (window.testRunner) {
        testRunner.dumpAsText();
        document.getElementById('layers').innerHTML = internals.layerTreeAsText(document);
    }
}
</script>
<!-- There should be a layer for the iframe's scroll corner alongside the iframe's content's layer tree -->
<body onload="test()">
<iframe class="container" src="resources/composited-subframe.html"></iframe>
<pre id="layers">Layer tree appears here in DRT.</pre>