In Mac OS X, the NSSplitView is transparent, and doesn't draw any
sort of background. It only draws the splitters.
On GNUstep, it looks to me like it draws a background by default, and
this color seems to be the same color as the window background. It
has the functions -[NSSplitView setBackgroundColor:] and
-[NSSplitView setDividerColor:].
I was wondering if it might be acceptable to alter GNUstep's
NSSplitView to by default not draw a background color (until it is
set with one of the setBackgroundColor options, considering that the
color is the same as the window not many people will notice unless
they have a custom window background color). It would, then, be in
alignment with Mac OS X's implementation (which I know it not the
goal to be 100% the same, it is just a discussion topic).
On a side note: I also filed a 'bug' report that brought attention to
the fact that NSSplitView on GNUstep does not implement (or dummy) -
[NSSplitView setDividerStyle:] (something else that would be nice to
see implemented (at least to be there but ignored).