bug#28695: 26.0.60; Rendering lag spikes caused by double-buffering on Linux

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 28695@debbugs.gnu.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Thu, 5 Oct 2017 16:36:53 +0300
> 
> Eli, perhaps it would be wise to ask the reporter if he'd like it to be 
> turned off by default, if this problem can't be fixed in time.

I kinda did, by proposing the recipe to turn it off, and waiting for a
response ;-)

> Personally, I can stand a few minor glitches, given that double 
> buffering saves me from flickering, which has been an annoyance for years.

I know that you are very happy with this feature, which is why it
puzzles me how come you (and others who like it) don't see these
problems.

> I wasn't able to compare with the revision before double buffering, 
> though: after checking out c29071587c64efb30792bd72248d3c791abd9337^, 
> ./autogen.sh says:

It should be easier to use a previous release of Emacs, if you have it
or can install it.

Btw, the hardships of building an old enough checkout are the reason
why I keep old binaries around, and insist on not doing a bootstrap
(which nukes them).  I find this a much easier way of "bisecting", or
at least of having a good start point for looking for a change that
causes some bug.