How did a user manage to get my multi-window app to run in "Tabbed Windows Mode"? Today one of my users sent me some screenshots showing my application running in "Tabbed Windows" mode, i.e. with all the usually-separate windows showing up as tabs inside a single window, like this:



This is very cool, especially since I never actually programmed in any support for a "tabbed windows mode" in an application.  :)

Unfortunately the user doesn't know how he got the application into this mode, and I can't figure out how to do it either.

When I Google the subject, I find articles like this one describing how this mode was added to some of Apple's own apps, and how MacOS/X will automatically added to any third-party apps that are "document-based"; for those apps, some additional menu items (like "Merge All Windows" and "Show Previous/Next Tab" and "Move Tab To New Window") will get automatically inserted into the Window menu.

However, my app isn't "document-based" AFAIK (it's a cross-platform app based on Qt5.4.1), and it doesn't have any menu named "Window".  (It does have a menu named "Windows", but those menu items do not show up in that menu)

Does anyone know how my user pulled off this trick?
Just wondering what version of macOS you're running? The other user must be using macOS Sierra, what about you?

In macOS Sierra tabs are supported in any application where it’s possible to have multiple windows open. Based on your description, your app supports multiple simultaneous windows. 

Assuming you have macOS Sierra installed, you can activate this in the way you found through your Google search. But if you haven't, then you won't be able to get tabbed windows to work.

If you do have the latest version of macOS Sierra installed, update your question and let me know. I'll see what I can dig up.