<style>
#headernavbar {
    position: fixed;
}
#system {
    position: absolute;
    visibility: hidden;
}
</style>
</head>
<body>
<div id="headernavbar">
<div>
<div>There should be text below</div>
<div id="system">
This should be visible
</div>
</div>
</div>
<script>
document.body.offsetWidth;
system.style.visibility='visible';
</script>