Ok, but the patch [here](https://gitlab.gnome.org/GNOME/gtk/-/issues/5397) is for an issue in Scite.
The commonly used versions of SciTE run directly on Win32 and macOS APIs which allows richer platform integration like supporting the MSDEVColumnSelect clipboard format on Win32 or dragging 'ghost' text on macOS. Therefore, SciTE for GTK's behaviour on Win32 or macOS isn't very important and I don't test it. It may work and I'll accept patches but its not mainstream.
Its a patch for Scintilla as used in Scite to fix a problem for GTK on Wayland on Linux, not GTK on win32 or macos, but as I pointed out may be superseded by the patch to GTK.
For Geany, running on Win32 or macOS means running through GTK and its wrapping of platform APIs so there is much more scope for problems to occur in Geany with these changes.
Sure, and it will get tested on Geany eventually but "somebody(s)" have to do it, as you probably know Geany contributors have little time available to work on it, and so it will take a while, especially for Windows and Macos platforms.
So in the meantime I was just pointing out the fix to Scintilla for Scite in case you wanted to add it for Linux to avoid problems until the GTK fix is distributed. It may also fix other issues with apps which only accept MIME types that have been raised.