Theming is internal, not public API though. It is fine to break internal APIs, that's why they are internal. Themers were warned, that the styling engine was u

Theming is internal, not public API though. It is fine to break internal APIs, that's why they are internal. Themers were warned, that the styling engine was under heavy rework. Gtk people could either wait few years until it's done and be stuck with Gtk2 meanwhile, or go out with Gtk3, which has relatively stable public API and internal API under rework. It's no brainer to take the same way they took. They also communicated this, so anyone who ignored their advice has no basis to complain. They were told exactly what is going to happen, and that they will either have to keep up, or not do it yet. Switch to client-side decorations was also done for a very good reason. There's a post by Rasterman (author of the Enlightenment WM) on reddit, where he details it why - and he is not even associated with Gnome. Windows and Mac also do client-side decorations, you just cannot see it, because clients cannot connect directly to the compositors, they have to use the client side libraries that handle that. Which is exactly what Gtk does.