@sagarchalise That plugin relies on geany providing the functions `sci_set_selection_mode` and `sci_get_selection_mode`, that is geany already supporting rectangle selections. I suppose geany could expose the underlying functionality from scintilla for plugins and just not in its own GUI and let a plugin handle it. This might actually be a good way to get testing for multiple selection corner cases: users can opt-in by installing the plugin, with the expectation that some things might be weird/broken. Eventually multi-edit-mode could be moved into geany proper.
I'm not sure how to handle the mouse portion in this scenario though.