Prior to macOS Sierra I would use C-x 5 2 to create a separate frame which was created in an OS X window so that there were now two windows. This is using the latest EmacsForOSX version straight out of the box. Life was good.

I recently switched to macOS Sierra and discovered there is now a different behavior: after C-x 5 2 the new window is turned into a tab inside the original Emacs frame (macOS window). An invisible frame at that if you are using macOS fullscreen mode. That sort of sucks.

Edit: Note that this behavior only exists in full screen mode so if you exit full screen mode C-x 5 2 does the right thing.

After playing around with the new macOS tab features I discovered that I could exit fullscreen mode to reveal the tabs and drag a tab out to create a separate Emacs frame. Life is better, however this is a rather clumsy way to get separate frames, for example, placed in each monitor on a dual monitor setup. Is there an easier way to achieve the pre-Sierra Emacs frame behavior in a post-Sierra world?

It strikes me that macOS Sierra breaks the legacy Emacs design decision that an Emacs frame can be modeled as an OS X/macOS window, therefore requiring code changes in Emacs to support the new macOS tab feature. Is anyone aware of Emacs development efforts to accept and deal with this?

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    I don't think anybody is working on this. However, you can post your question on emacs-devel to make sure. Of course, contributions are always welcome! Jul 4, 2017 at 10:00
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    Does this still happen if you start Emacs with -Q? I'm on Sierra using Emacs 25.2.1, and if I do C-x 5 2 I end up with two Emacs frames just as I have always done.
    – stevoooo
    Jul 4, 2017 at 11:37
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    Please consider posting an answer to your own question, if you have found one. E.g., let us know what it was in your init file that caused the problem. That might help others.
    – Drew
    Jul 4, 2017 at 14:39
  • @Drew why do you choose "mac OS" over "macOS" which appears to be the Apple preferred spelling? Just a personal preference or is it based on some generally accepted document?
    – pajato0
    Jul 6, 2017 at 7:04
  • @Drew, yes, I will post an answer once I understand the aberrant/changed C-x 5 2 behavior in Sierra.
    – pajato0
    Jul 6, 2017 at 7:10

2 Answers 2


Matthew Bauer (from the Emacs team provided this answer)

Prior to macOS Sierra, in full screen mode, C-x 5 2 would create the new Emacs frame in a new OS X window. This is actually configurable within macOS settings. To accomplish this go to the menu bar:  → System Preferences... → Dock → Prefer tabs when opening documents and select "Manually". Of course this will change the behavior in other apps when in full screen.

fwiw, the Emacs developers are wrestling with a different solution but whatever that may be will not likely be available for months.


Since does not always make sense for the default behavior of Preview and Emacs to match, the following lines in ~/.emacs.d/init.el will prevent tabs from being created.

(setq mac-frame-tabbing nil)

There are also mac-move-tab-to-new-frame and other mac-... functions that deal with native tabs.

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