I thought using Alt + Arrow keys (up, down) should emulate mouse wheel behaviour
Where did you get that impression? I can't find anywhere that exactly defines what that key combination does, but nothing I can find says its a mouse replacement.
All the [Geany docs](https://www.geany.org/manual/current/index.html#editor-keybindings) say is "scroll view" with no mention of what the cursor does or does not do, unlike the line above for `ctrl_shift+l`. The Scintilla docs are similarly silent (which is probably why Geany ones are, since Geany simply calls Scintilla).
Certainly the oldest version I have (1.36) works like that, and as far as I can recall it always did, but I never paid much attention.
This is also the way Scite behaves (except its key combo is different).
Certainly it is strange, exactly what it is for I don't know.
Is this intended behaviour?
You would have to ask that of Scintilla, perhaps there is some olden times use.