Bug filed id:4742. In the meantime one possible workaround you might try for now is to allow the shell to be 100% size (i..e default) and reparent the body elements to the shell's div. This may not be practical for you depending on how complex the body element is. Though you may be able to wrap the top level elements in a div
e.g.
<body>
lots of elements
</body>
<body>
<div id="someId">
lots of elements
</div>
</body>
Then reparent "someId" to the shell's div?