We still occasionally receive bug reports about crashes on macOS
10.12+ caused by macOS turning fullscreen windows into ‘tabbed’
windows. I’ve also seen it discussed in a few places round the web.

We currently have a compile‐time check to include code that fixes this
on 10.12+, and it can be over‐ridden if Emacs is being built on an
older version of macOS but is expected to run on 10.12+, however it is
clearly not doing the job.

I propose for Emacs 26.2 (and the master branch) that we remove the
compile‐time check and just use a run‐time check. This will result in
a compiler warning on macOS 10.11 and below, but will prevent crashes
everywhere.

Any thoughts?

Patch attached.
-- 
Alan Third