<!DOCTYPE html>
<style>
#container {
    width: 100px;
    height: 100px;
}

#child {
    position: fixed;
    right: 0;
    bottom: 0;
    width: 100px;
    height: 100px;
    background: green;
}
</style>
<div id="container">
    <div id="child">
    </div>
</div>
This test verifies changing CSS containment correctly invalidates layout.
<script>
    var container = document.getElementById("container");
    container.offsetTop; // forces layout
    container.style.contain = "paint";
</script>