bug#61667: 29.0.60; Failure to redisplay

[Dmitry Gutov]

On 16/04/2023 21:50, Eli Zaretskii wrote:
> > Date: Sun, 16 Apr 2023 20:46:39 +0300
> > From: "Dmitry Gutov" <dgutov@yandex.ru>
> >
> >   When you say "gets updated noticeably faster", what exactly do you
> >   mean? by how much time does the redraw of the windows lag after the
> >   title bar update?
> >
> > Can be on the order of seconds or indefinitely, just like described before.
>
> That's strange.  Po Lu, what could prevent the results of rdisplay
> from appearing on the glass in this case?  AFAIR, we simply call
> XFlush when the update of the frame is done; we don't wait for any X
> event to trigger redrawing on the glass.
>
> Dmitry, does this happen with Emacs 28?

Actually, this one might be a master-only problem. At least, I'm 
currently unable to reproduce this with emacs-29, and (probably) saw 
this only once on emacs-28, out of >50 tries. All with Lucid and xdbe=off.

But it's pretty easy to repro on master. I'll try to bisect and report back.

BTW, I forgot one step which is possibly necessary for reproduction 
(step 1.5): maximize the frame.