Oh my gosh. @djohnston! It’s almost perfect!
In playing with this build, I realized that if I have the app full screened (my fave), and I hit super+n, I get a new window, and it automatically combines with (Mac) os-native tabs. I can also do super+t for a sublime-native tab (which I personally don’t really prefer, although tabs on tabs does have some interesting opportunities 
). P.S. this is present in build 2000 as well.
The only thing missing IMO (and believe me, I don’t mean to come across as implying anyone is my own personal dedicated dev team lol!) when I quit and reopen Sublime Merge, the tabs are not remembered (unlike SublimeText3 
) so the fullscreen (which is remembered) sorta fails because it only can do one, and you can even hear the OS give a “bonk” noise because something didn’t quite work (presumably the full screening of what was expected to be a single window, instead all the tabs got pulled apart into separate windows so the OS doesn’t really know how to proceed).
Anyhow, killer work you all, and thanks for listening to user feedback. This is a kick-a$$ product. So fast, so clean. I’d love to hear your thoughts on fullscreen/tab functionality. Is there anything that could be pulled in from ST3 so it “just works”? Keep up the good work!
Edit: Looks like this is already logged (#450) (and I even commented on it 
).