<!DOCTYPE html>
<html>
<head>
<style>
    .container {
        position: absolute;
        top: 100px;
        overflow: hidden;
        width: 500px;
        height: 150px;
        border: 1px solid black;
    }

    .mover {
        position: absolute;
        width: 500px;
        height: 100%;
        left: 100%;
        transform: translateX(-100%);
        background-color: silver;
        padding: 10px;
    }
    
    .mover.animating {
        animation: slide 1s linear forwards;
    }

    @keyframes slide {
        100% { transform: translateX(0) }
    }
</style>
<script>
    if (window.testRunner) {
        testRunner.dumpAsText();
        testRunner.waitUntilDone();
    }

    function dumpLayerTree()
    {
        if (!window.internals)
            return;

        var out = document.getElementById('out');
        out.innerText = internals.layerTreeAsText(document, internals.LAYER_TREE_INCLUDES_BACKING_STORE_ATTACHED);
    }

    function dumpLayersSoon()
    {
        requestAnimationFrame(function() {
            // Trigger layer flush
            document.querySelector('.container').style.width = '501px';
            dumpLayerTree();
            if (window.testRunner)
                testRunner.notifyDone();
        });
    }

    window.addEventListener('load', () => {
        let box = document.getElementById('box');
        box.addEventListener('animationstart', dumpLayersSoon, false);
        box.classList.add('animating');
    }, false);
</script>
</head>
<body>
<pre id="out"></pre>
<div class="container">
    <div id="box" class="mover">
        Some text here to force backing store.
    </div>
</div>
</body>
</html>