bug#52085: 29.0.50; Frame occasionally fails to paint while screen sharing on macOS

[Eli Zaretskii <eliz@gnu.org>]

> From: Aaron Jensen <aaronjensen@gmail.com>
> Date: Wed, 8 Dec 2021 16:33:06 -0500
> 
> > To be honest, if I just want to see when one function is called I
> > stick in 'NSLog (@"Moo")' or whatever. There's too much output in
> > NSTRACE and I'm sure that sometimes it doesn't print out results from
> > things that are called deep in the toolkit code.
> 
> Well, I can say with some certainty that those messages are the only
> ones appearing under typical operations like key presses. I can insert
> newlines into the terminal and observe that only those are emitted
> when I press keys or even scroll. So the display function is not being
> called unless there is some other wildness going on with the tracing.
> Do you expect it to be called every render? (It is also not called
> when I run M-: (redisplay))

I'm not tracking this discussion, so apologies if what I say below
makes no sense.  However, the Emacs display engine avoids redrawing
anything if it concludes that what's on the glass is correct.  So just
calling 'redisplay', or making some simple changes that don't require
significant redrawing, don't necessarily result in calls to low-level
drawing functions, you need to do something drastic to force those
with 100% certainty.

HTH