doesn't work for me in geany 3.18 on gtk3.24.31 (on F36 gnome/wayland)
To summarise, Wayland changed the names of the data types (to `text/plain` `text/plain;charset=utf-8`) for cut and paste, so Scintilla and other applications that only understood the X11 names (`STRING` and `UTF8_STRING`) didn't work on Wayland when transferring to any application that did not accept/provide the olde X11 names.
A fix was added to GTK3 to translate X11 names to Wayland names and that was included in 3.24.35 Nov 23 although bleeding edge distros that used pre-releases may have seen it before then. Unfortunately that fix was not [complete](https://gitlab.gnome.org/GNOME/gtk/-/commit/0c1ea9221976b838101d2593754b6588...) so totally broke cut and paste between old applications and new ones on Wayland.
The fix to the fix was merged Dec 8 and is in 3.24.36 which should make all cut and paste work for olde apps (eg any Scintilla based app) under Wayland. If that is not the case please report to Scintilla and GTK.