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?