Drawing to screen - nothing appears?

[Steven Schlansker]

I'm trying to port an application that I've developed on Mac OS X to 
GNUStep/Linux and GNUStep/Windows.  I'm currently running Gentoo with
gnustep-base/gnustep-base-1.10.2_pre20041116 
gnustep-base/gnustep-back-xlib-0.9.5_pre20041116 
gnustep-base/gnustep-gui-0.9.5_pre20041116

I'm using the libart backend, but xlib doesn't work either.
The method I use for drawing goes like this:

[NSWindow lockFocus]
[NSColor set]
NSRectFill (to clear the screen)
for()
   [NSColor set]
   NSRectFill (to draw object)
[NSWindow unlockFocus]
[NSGraphicsContext flushGraphics]

Usually nothing shows in the window that I've created.  Sometimes a 
rectangle appears as it should, but I can't figure out what triggers 
this or why, and it disappears on the next refresh.  It works fine on 
Mac OS X.  Is there something that I'm doing wrong?

Thanks for any pointers (except for nil ones ;) ),
Steven Schlansker