It's entirely possible where in Catalyst if a UIKeyCommand is registered that uses a non-modified letter, it overrides text input. Having said that, I can't imagine why Twitter for Mac would have solo-letter key commands registered during the login flow, and if they did then I can't imagine why they wouldn't also interfere during tweet composing. Or rather, I can imagine, but the scenarios I'm constructing would imply extremely contorted view controller hierarchies that make no sense.