Windows 7 SP1 64-bit, Geany 1.23.1, fresh install from the Windows binaries, including GTK+ 2.16 (but the problem also occurred on 1.23).
AMD A6-3650 APU, 2.6 GHz, 4GB RAM (I've had some strange things with this APU before, so I thought I'd mention its an APU, not a CPU...perhaps it matters)
For any text file being edited, old or new, any type, any encoding, I cannot enter characters using Alt+(ASCII code) or Alt+[00000-65536].
I CAN however enter these characters in some other editor, and copy-paste them over into Geany.
Really nothing seems to happen when entering text this way; when typing some Alt-(ASCII code) in a new blank file, the file's status does NOT change to having "unsaved changes".
Likely related: I have a bunch of AutoHotkey scripts, which replace some code I type with a (Unicode) character (for example, typing the string "\omega" will be replaced by "ω". The following snippet in the .ahk file accomplishes that: ":?*oc:\omega::ω" (without quotes). Note that the .ahk file should then be saved in UTF-8).
This works great everywhere in Windows, except in Geany; the typed code is correctly deleted (the "\omega" part), but NOT replaced with the desired character. This seems to be the case for both Unicode characters and ASCII characters (I also have a "\grave" autotext, which should be replaced with the grave accent ("`", ASCII 096)).