fix menubar height measurement

we were measuring the wrong divs,
causing the pager splitter to occasionally be off-screen,
or the top ~8 pixels of the header to be cut off
(e.g. when visiting a url with an anchor).
Merge pull request #4384 from minrk/height

fix menubar height measurement

we were measuring the wrong divs, causing the pager splitter to occasionally be off-screen, or the top ~8 pixels of the header to be cut off (e.g. when visiting a url with an anchor).