bug#57434: 28.1.91; Terminal Emacs Mac OS flickering.

[Eli Zaretskii <eliz@gnu.org>]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Cc: Dmitrii Kuragin <kuragin@google.com>,  57434@debbugs.gnu.org
> Date: Thu, 01 Sep 2022 07:44:52 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > Given our inability to pinpoint the root cause of the problem, and the
> > unsolved mystery why on some macOS systems the problem is much more
> > prominent than on other macOS systems, I tend to introduce a variable
> > that users could set from Lisp to tell Emacs not to use the optimized
> > insert/delete-lines algorithm in scroll.c.  Unless some significant
> > new ideas or facts emerge in the next day or two, that is.
> 
> I guess I've found these facts today, while rummaging in alacritty's
> Github project.

Thanks.

So we have 2 alternatives:

  . declare that flickering on alacritty is currently a known problem,
    and wait till Someone comes up with the proper solution for Emacs;
  . add that variable I mentioned above, and let users try to fix the
    problem with alacritty by flipping it.

TBH, given your description, I'm no longer sure a simple boolean
variable that disables insert/delete-line optimization will do, since
the issue seems to be a much more general one.  How sure are we that
some other scenario of redrawing a TTY frame won't cause similar
flickering regardless of the insert/delete-line feature?  So maybe
just document the issue in PROBLEMS and advise against using
alacritty?