On 6 Jul 2017 12:29 p.m., "Paul Michael Reilly" <pmr@pajato.com> wrote:

I have recently switched to macOS Sierra and discovered that using C-x 5 2
to create a new frame works just fine, i.e. no different than OS X, if you
do not enter full screen mode (via the green window decoration button).  If
you are in full screen mode C-x 5 2 is ignored, but if you then exit full
screen mode it is clear that the second frame is now a tab (macOS Sierra
tab feature).


This behaviour should be fixed in master. Can you please try it? (nightly
builds if you're using emacsformacos.com builds)

Thanks.