bug#30088: Rendering glitch with double buffering enabled

[Po Lu via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Eli Zaretskii <eliz@gnu.org> writes:

> 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.)

Yes.  This particular bug also depends on the video driver and X server
to a larger degree than normal.  Damien could have upgraded his X
server, but other people might be running different versions (or
entirely different servers altogether.)