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

[Artyom <yom@artyom.me>]

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.

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)

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.

I triedcompiling Emacs from source (August 19, commit 4a80c8bb) so that
it would use my version of GTK (I have 3.20.9, Emacs in my distro's
repository was compiled with 3.16.16), but that didn't fix anything.

In GNU Emacs 24.5.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.16.6)
of 2015-09-10 on foutrelis
Windowing system distributor `The X.Org Foundation', version 11.0.11804000
System Description: Arch Linux

Configured using:
`configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
--localstatedir=/var --with-x-toolkit=gtk3 --with-xft
'CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong
--param=ssp-buffer-size=4' CPPFLAGS=-D_FORTIFY_SOURCE=2
LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro'

Important settings:
value of $LANG: en_US.UTF8
locale-coding-system: utf-8-unix

Major mode: HTML

Minor modes in effect:
tooltip-mode: t
electric-indent-mode: t
mouse-wheel-mode: t
tool-bar-mode: t
menu-bar-mode: t
file-name-shadow-mode: t
global-font-lock-mode: t
font-lock-mode: t
blink-cursor-mode: t
auto-composition-mode: t
auto-encryption-mode: t
auto-compression-mode: t
line-number-mode: t
transient-mark-mode: t

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Mark set
Quit [2 times]
Mark set
Quit
Mark set
scroll-down-command: Beginning of buffer [16 times]

Load-path shadows:
None found.

Features:
(shadow sort gnus-util mail-extr emacsbug message format-spec rfc822 mml
easymenu mml-sec mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums
mm-util help-fns mail-prsvr mail-utils sgml-mode time-date tooltip
electric uniquify ediff-hook vc-hooks lisp-float-type mwheel x-win x-dnd
tool-bar dnd fontset image regexp-opt fringe tabulated-list newcomment
lisp-mode prog-mode register page menu-bar rfn-eshadow timer select
scroll-bar mouse jit-lock font-lock syntax facemenu font-core frame cham
georgian utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao
korean japanese hebrew greek romanian slovak czech european ethiopic
indian cyrillic chinese case-table epa-hook jka-cmpr-hook help simple
abbrev minibuffer nadvice loaddefs button faces cus-face macroexp files
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote make-network-process
dbusbind gfilenotify dynamic-setting system-font-setting
font-render-setting move-toolbar gtk x-toolkit x multi-tty emacs)

Memory information:
((conses 16 83794 8536)
(symbols 48 17775 0)
(miscs 40 48 138)
(strings 32 10734 4394)
(string-bytes 1 276508)
(vectors 16 9126)
(vector-slots 8 386896 15317)
(floats 8 64 561)
(intervals 56 3496 6)
(buffers 960 12)
(heap 1024 9055 942))