The documentation says that Ctrl+Shift is used for rectangular selection on Linux but it seems to be Alt+Shift. See https://github.com/geany/geany/issues/4219.
(Note that I'm using Linux in a VM on macOS so I'm not 100% sure how it behaves on normal Linux.) You can view, comment on, or merge this pull request online at:
https://github.com/geany/geany/pull/4222
-- Commit Summary --
* Fix documentation of rectangular selection
-- File Changes --
M doc/geany.txt (4)
-- Patch Links --
https://github.com/geany/geany/pull/4222.patch https://github.com/geany/geany/pull/4222.diff