As I said above I couldn't find where Geany does anything with the down arrow key, AFAICT if it gets one, it just checks its not something it cares about and then passes it to Scintilla.
You can see in `keybindings.c` where Geany defines scintilla to handle `ctrl+down` and `alt+down` but nowhere is plain `down` defined or undefined by Geany that I can find, so it will be the Scintilla default.