> But there is an issue with Win32 API programming. And the truth is that custom windows mean doing everything yourself, controlling every Windows message, and that is fragile

This isn't actually true though. You can delegate to the default window proc, and only customise what you want.

Sure, if your window is now a triangle, you need to think about how resizing is going to work. But you don't need to re-implement everything from scratch -- only the defaults that aren't compatible with your new design.