bug#24267: 24.5; When scrolling, parts of buffer are redrawn with a delay

[Eli Zaretskii <eliz@gnu.org>]

> From: Artyom <yom@artyom.me>
> Date: Fri, 19 Aug 2016 23:48:17 +0300
> 
> When I scroll, most of the buffer redraws immediately, but a few
> lines at the bottom of the buffer only redraw after a noticeable
> delay (200ms or so, I would guess). You can see a screen recording
> here:
> 
> http://files.artyom.me/emacs.mkv
> 
> Steps to reproduce:
> 
> 1. emacs -Q
> 2. Open a file with lots of syntax to be highlighted (for instance,
>     just save https://en.wikipedia.org/wiki/Emacsas an HTML page
>     and open it in Emacs).
> 3. Start scrolling with C-v, and wait a couple of seconds after
>     each scroll.

I cannot reproduce this: I don't see these redraws at all, using the
HTML page you pointed to.

It could be some issue specific to the default font you use (you don't
say which font is that).  Or some other issue local to, or triggered
by, your local setup.  Because this:

> It's easiest to reproduce with a big file with syntax highlighting
> enabled, but it also manifests on many other occasions:
> 
>    * When scrolling an ordinary textfile(with font-lock disabled)
>    * When pressing Enter in a buffer, forcing other lines to shift
>    * When switching the focus from one buffer to another,
>      cursor moves only after a delay
>    * Sometimes parts of selection don't disappear after C-g
>    * Sometimes parts of the buffer that take longer to redraw
>      are just few characters in the middle of a line (i.e. it's not
>      necessarily a prefix/suffix of visible text)

actually says the problem should be highly visible, and yet it was not
reported until now, at least not with such intensity.

I cannot play your screencast here, so I'm not even sure that when you
talk about redrawing you mean the same thing I do.  Therefore, one
possibility is that something is wrong with your X setup or even with
your video hardware and software.

> This all only started happening after I upgraded my system.
> Unfortunately, the upgrade was pretty huge and so it's hard to
> pinpoint a specific package that caused this. The version of Emacs
> stayed the same.

Which is a serious argument in favor of my theory that this is not an
Emacs problem.

Maybe someone else bumped into such problems and could advise you
where to look.

Thanks.