After the fiasco of sharing a single NSWindowController among multiple NSWindow instances, one per tab, let’s look at a proper implementation. For the working example, see the code on GitHub.
This is a follow-up to “Programmatically Add Tabs to NSWindows without NSDocument” from January this year. There, I was creating NSWindow instances from storyboards and re-used a single window controller to manage them. The point of that post was to solve the problem that newly created tabs will themselves not respond to the “+” button – or any menu action, for that matter, because they fell out of the responder chain at first.
The Cocoa/AppKit documentation is very sparse when it comes to tabbing. You can make use of the native window tabbing introduced in macOS Sierra with a few simple method calls, though. Conceptually, this is what you will need to do: However, there are some caveats when implementing these methods naively. The plus button may stop working (no new tabs are added when you click it) and all default shortcuts are broken, their main menu items greyed out.