Are you confused about rectangular selection, its https://www.scintilla.org/ScintillaDoc.html#SCI_CHANGESELECTIONMODE not multiple selections.
No, I'm talking about the fact that if we want to have multiple carets on Alt+click, it would be also good to have rectangular selection on Alt+Shift+click so one doesn't have to remember which of these multi-caret modes is alt and which one is ctrl. At the same time though, we should probably support the existing Ctrl+Shift+click keybinding because existing Geany users are used to it already. That's why I suggested `SSM(sci, SCI_SETRECTANGULARSELECTIONMODIFIER, SCMOD_CTRL | SCMOD_ALT, 0)` for the rectangular selection (which doesn't work right now but I had a look at the Scintilla code and it should be quite trivial to modify it).