I think my favorite cross-platform GUI library is wxWidgets, because you can manually construct an interface from nested linear layouts, which is the way I prefer to construct my interfaces.
Apple seems to have a much more powerful constraint system for this sort of thing, but I haven't taken the time to learn it yet, so I can't speak to whether I'd prefer it or not.
Apple seems to have a much more powerful constraint system for this sort of thing, but I haven't taken the time to learn it yet, so I can't speak to whether I'd prefer it or not.