Steven, you just drag a "custom view" into the instances tab of the nib contents window. Those views aren't sitting in windows, they're just freestanding views. IB shows them in windows for convenience but they only show up in the app because they get added as subviews.

Mr. Noodle, thanks for this. I was not aware of NSView's -setAutoresizingMask:, and I'm sure it'll come in handy.

A related example is Apple's "Reducer" demo code, which includes an NSTabView subclass that animates transitions between its views. It's on Apple's site at http://developer.apple.com/samplecode/Reducer/index.html