<!DOCTYPE html>
<html>
<head>
<style>
body {
    height: 2000px;
}
.masthead {
    position: sticky;
    width: 200px;
    height: 100px;
    background-color: rgba(0, 0, 0, 0.2);
}

.make-fixed .masthead {
    position: fixed;
}

.fixed {
    position: fixed;
    width: 100px;
    height: 100px;
}
</style>
<script>
    if (window.testRunner) {
        testRunner.dumpAsText();
        testRunner.waitUntilDone();
    }

    function doTest()
    {
        window.setTimeout(function() {
            document.body.classList.add("make-fixed")
            if (window.testRunner)
                testRunner.notifyDone();
        }, 0);
    }
    
    window.addEventListener('load', doTest, false);
</script>
</head>
<body>
<div class="masthead">This test should not assert in debug.
    <div class="fixed">
    </div>
</div>
</body>
</html>