I see that the deeper you go, the less you can keep common. I learned a lot from your answers.

I didn't understand why the nativeness of the UI must bifurcate (fork) the codebase. In your example, can't non-modal on Mac vs modal on Windows be a config value in the cross-platform UI?

I also didn't get an answer to one of my questions. Why can't applications have low level access to create controls on their own?