This is rather a feature request than an issue. I'd welcome being able to edit multiple lines at once using multiple cursors.
Scintilla 4 can do that, but Geany doesn't use Scintilla 4 because it requires C++17, perhaps Geany should be branched into an "ancient_backward_luddite" branch like Scintilla is and then master canfollow modern Scintilla. Just needs somebody to do it.
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