The FLTK code currently cannot get the correct work area when you have more than one monitor active on X11. The reason is that it looks as _NET_WORKAREA, which was never extended to properly handle multi-head.
Qt fixed this by enumerating over all panels and calculating the area manually:
We can probably do something similar.
We now care a lot less about the work area since r37660 for bug 7793. It is now used for placing the initial vncviewer window on the same monitor as tlclient, and to make sure the initial vncviewer window doesn't overflow the screen.