Thank you Alan for taking a look at this thread.  Here is the code ...

Using a build of the master branch as of today (10/12/2018):

git reset --hard 9c231a447014823ed1955e16b6693adbe041ca99

git apply /path/to/attached/patch.diff

Build a GUI version of Emacs --with-ns

Open up the newly built Emacs and type:

M-x mc-test

or

M-x +-mode

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> Date: [10-12-2018 01:54:15] <12 Oct 2018 09:54:15 +0100>
> From: Alan Third <athird@googlemail.com>
> To: Keith David Bershatsky <esq@lawlist.com>
> Cc: Emacs-Devel devel <emacs-devel@gnu.org>
> Subject: Re: nsterm.m: How to play nice with Mojave revisions -- #17684 (crosshairs) and #22873 (multiple fake cursors) ...
> 
> * * *
> 
> Hi Keith, this looks a bit strange.
> 
> The idea is that when running redisplay cocoa does not let you draw to the screen, so ns_clip_to_rect, etc., marks the area as needing to be redrawn later, and returns
> NO so you know not to try drawing to the frame.
> 
> Later drawRect is called with a list of the areas that have been marked as needing redrawn and it calls expose on those areas and so Emacs runs the drawing functions
> again, but this time ns_clip_to_rect returns YES so you know you can go ahead and draw.
> 
> For some reason NSTRACE doesn't show the functions being called from within drawRect, so if you're relying on that it may mislead you.
> 
> I don't know what's going on with your code, is there somewhere I can have a look?