This will be a Sassy jQuery plugin for off-canvas content. It won't work without Sass, that's the only sane way to do it.
<body class="off-canvas">
<ul class="off-canvas-menu">
<!-- mobile navigation or what have you -->
</ul>
<div class="off-canvas-visible">
<!-- visible until the breakpoint -->
</div>
<div class="off-canvas-hidden">
<!-- hidden until the breakpoint -->
</div>
<div class="off-canvas-content">
<!-- pushed away when the menu opens -->
</div>
</body>