Seriously though, a stable API is not the GTK/GNOME developers’ agenda here. Nobody wanting a stable API should write software with this toolkit.
This blog post doesn’t mention GTK, but I’ve heard GTK will sometimes implement breaking changes in minor version bumps. I was thinking about writing some software with GTK, and I haven’t been deterred so I guess I’ll learn the hard way, but has GTK 4 had any of these stability problems yet?