This is the fault of the GTK Look and Feel that is used by default in Ubuntu. You have two options:

1) Use Ctrl-Space. This will hide the 3D pane effectively. I can't find a shortcut to bring it back, so you would have to drag it up. There might be one, I just can't find it right now.

2) Disable the GTK theme so you see the arrows on the splitter. Warning: Ugly UI ahead! Just add the following comand line option in the SweetHome3D startup script: