After using the program for a while, I found that moving the window to a larger screen and maximizing it, I found the Variable pane by accident by noticing that the mouse cursor changed shape when moving it to the screen edge. This made it possible to pull out the Variable pane, which seems to be a split of the Context tab.
The bug is thus that the Variable pane is completely invisible by default, not that it does not exist. Since Nemiver opens up with a default window size, and the pane was not visible anywhere, I still consider it a bug, though the underlying issue may lie in Gnome and/or GTK.
