I don’t know why or how, but using gtk_window_set_geometry_hints() has the desired effect (now?). Reading GTK’s source code is too convoluted to find out what’s going on here. I can’t find a corresponding Wayland protocol.

Sway gets very slow when resizing such a window, so I’m a bit inclined to think that GTK does some weird trickery to get this to work. 🤔

Either way, xiate now sets geometry hints again and floating windows have the correct size now. Finally.