bug#30088: Rendering glitch with double buffering enabled

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: Eli Zaretskii <eliz@gnu.org>,  Dmitry Gutov <dgutov@yandex.ru>,
>   30088@debbugs.gnu.org,  charles@aurox.ch,  npostavs@users.sourceforge.net
> Date: Mon, 21 Feb 2022 17:04:19 +0100
> 
> Damien Cassou <damien@cassou.me> writes:
> 
> > I have double-buffering back on and saw no glitches for years. I think
> > the issue might safely be closed. We might even remove the related
> > section in PROBLEMS.
> 
> Yup; now done (on master).

I reverted that commit.

PROBLEMS is special in that it describes issues with software other
than Emacs.  Since we cannot control that external software, we
usually cannot know whether and when those issues are resolved, nor
whether old versions of that software are still being used by someone
who happens to run a newer Emacs.  So the fact that Damien no longer
sees the problem doesn't mean it cannot happen to someone else,
because what fixed that for Damien could be upgrade of some software
(other than Emacs) which other Emacs users don't necessarily share.

For this reason, we almost never can positively know that a PROBLEMS
entry is no longer relevant, and that is why we have there very old
entries.  (It's possible that some of them can be removed, but the
rationale should be that we can establish positively that the software
causing problems is known to not be relevant anymore.)