It means that GTK itself (internally, not necessarily its API) has warts.

Very doubtful to me.

The design of gtk is to separate the rendering in the GDK layer. Prior versions ran on Win32 without application changes.

That's how you can get it on X11 or Wayland without changing applications. Wayland didn't exist when this design choice was made, but has benefited from that choice. Imo rather than a liability, this is what good design looks like for such a library.

No doubt. But isn't that unrelated to what I said? I was just explaining my grandparent's comment.

To provide the same API for such a broad set of underlying technology, you necessarily have to shoehorn a few things to make them fit. That doesn't make the API bad. It's awesome that applications don't have to do this on their own!

Yes. I've seen it too often be the case that an appropriate layering or abstraction is derided as too much complexity, and that you should replace it with straight calls into the dependency. The former is often a strength, not a weakness!