I'm trying to write a GTK+-based UI for my favourite terminal-based editor, Kakoune. Kakoune heavily uses its own completion menus, so I couldn't really use the existing GTK+ popup menus, but rather added my own that matched Kakoune's expectations.

Under X11, this is fine and works beautifully.

Under Wayland, well...

Wayland (or GDK's Wayland backend) tells every window its position is (0,0), so I can't make a menu change direction if it would pop up off the screen.
Wayland (or GDK's Wayland backend) always reports the full size of the monitor rather than cropping off the space used by launchers/docks/etc., so I can't prevent my menu from overlapping those either
Thanks to client-side decorations, when I ask "how big is my window" I get a rectangle whose origin is a little bit up and to the left of my actual drawing area, so my menu always appears a bit up and to the left as well.
GTK+'s built-in menus suffer none of these problems, because GDK provides a special function that does exactly what I want, but it's private and unstable so it's not listed in the documentation and I can't call it.

I really thought that since GTK+ 3.x was declared stable, it'd be ready and complete by now. Maybe GTK+ 4 will actually work for me?