I see these methods to fix this bug:
- Use undocumented GtkKeyHash, this is what powers GTK accelerators internally: [header](https://github.com/GNOME/gtk/blob/master/gtk/gtkkeyhash.h), [source](https://github.com/GNOME/gtk/blob/master/gtk/gtkkeyhash.c), [example](https://github.com/GNOME/gtk/blob/ab051fd53f8176665c36620a1616014e4c9a59df/g...).
- Add missing hotkeys to the AccelGroup and drop the custom hotkey handling in on_key_press_event.
- Remake all hotkey handling using [Accelerator Maps](https://developer.gnome.org/gtk2/stable/gtk2-Accelerator-Maps.html).
- Remake all hotkey handling using [Bindings](https://developer.gnome.org/gtk2/stable/gtk2-Bindings.html).
If nothing of these is planned for the next release, I suggest to stop enabling-disabling cut/copy actions on the Edit menu opening. It can be easily done by commenting out [ui_update_menu_copy_items](https://github.com/geany/geany/blob/65988f51f0634d29d4454602f367f03f925fc6d2...) and doesn't lead to any problems, only visually the menu entries will be enabled all the time. This won't make unbound hotkeys like "Go to matching brace" <Primary>b usable in non-Latin layouts, but will at least fix the confusing Ctrl+X, Ctrl+C behavior.