@dreamlayers nice analysis, probably right
AFAICT Scintilla only offers the X11 atoms [here](https://github.com/geany/geany/blob/96342f3eb94dc6d49a05a65c9d73becca9009aa0...), but it will also accept the mime atoms [here](https://github.com/geany/geany/blob/96342f3eb94dc6d49a05a65c9d73becca9009aa0...).
AFAIK no current core contributors are using Geany on Wayland, but maybe someone who is can try simply adding the mime types to the list in the first link above and see what happens. (I'm not a Scintilla/Wayland expert so no guarantees ;-)
I have also suggested [elsewhere](https://github.com/geany/geany-osx/issues/40) that Scintilla GTK should handle rectangular selections the same way it does on Windows (define its own target type) so maybe `ScintillaGTK.cxx` needs a bit of a general wash and brush up to handle modern systems. Just needs "somebody" to do it.