if this is something we want
As I said several times over the years on #1141 _so long as it is confirmed that no Geany or core plugin or GP plugin functionality breaks_ it can be introduced in stages, basic Scintilla provided capability, then expand Geany and plugin functionality that can sensibly apply to multiple selections to do so over time.
But I have to point out Ctrl+click is the multiple caret shortcut in Vscode on Linux not alt+click, maybe thats a Mac thing, and Scite is also Ctrl+click[^1].
So maybe click for one selection, Ctrl+click for multiple selections (after first), and Ctrl+shift+click for rectangular selection, all nice and consistent and in agreement with at least a couple of other apps, and no Scintilla patch needed.
[^1]: not a wide sample I agree, but its all I have on this machine, and I wonder where I got the idea it was alt+click?