Fixed prefill issue where content wasn't loading, even though document was larger than screen.

On my app, I have a page where the left-nav makes the document larger than the window.  However, the content pane in the right side is blank.

The needsPrefill function checks whether the document is smaller than the window; if it is, the code gets elements until the document is larger.  However, in my case, the document is already larger than the window.  The result is that I have a blank content div.

My change is to modify needsPrefill to use the content div's size, instead of the whole document size.