Scintilla will be changing the key/mouse binding for rectangular selection, as per:
https://groups.google.com/d/msg/scintilla-interest/kZcE8firI3A/dsBn0joZAwAJ
Since Geany doesn't support multiple selections, and almost every X11 window manager captures <kbd>Alt</kbd>+drag for moving a window (has saved my bacon on a number of occasions), before this change lands in Geany, I propose we change Scintilla's new default back to the existing behaviour. While it's annoying that the Windows and non-Windows behaviour would remain inconsistent, it will probably save a lot of bug reports about this feature not working, and avoid having to change any related documentation as well.
Ideally anyone interested would make this user configurable (if it is already, I couldn't find it) in the process.