When follow-mode is enabled and the displayed buffer ends before the last window, Emacs becomes extremely slow.
This broke on revision 115272 (see log below).
Steps to repeat the problem:
emacs -Q
C-u 10 RET
C-x 3
M-x follow-mode RET
Here, moving the cursor up or down one line takes about one second. Holding and the arrow keys cause the cursor to disappear until the key is released or the edge of the buffer has been reached.
The problem disappears as soon as some parts of the buffer is shown in the rightmost window.
I am the original author of follow-mode, so I can share one interesting implementation detail. When the viewed buffer ends before the last window, follow-mode tries to display this window without any content (by setting the window start to point-max). Unfortunately, the Emacs display engine always tries ensure that windows are not empty so it repositions it... So, follow-mode hammers in its view of the world every chance it gets, currrently in post-command hook and window-scroll-functions.
Sincerely,
Anders Lindgren
Ps. Log for revision 115272:
revno: 115272
committer: Stefan Monnier <monnier@iro.umontreal.ca>
branch nick: trunk
timestamp: Thu 2013-11-28 17:43:09 -0500
message:
Refine redisplay optimizations to only redisplay *some* frames/windows
rather than all of them.
* src/xdisp.c (REDISPLAY_SOME): New constant.
(redisplay_other_windows, wset_redisplay, fset_redisplay)
(bset_redisplay, bset_update_mode_line): New functions.
(message_dolog): Use bset_redisplay.
(clear_garbaged_frames): Use fset_redisplay.
(echo_area_display): Use wset_redisplay.
(buffer_shared_and_changed): Remove.
(prepare_menu_bars): Call Vpre_redisplay_function before updating
frame titles. Compute the actual set of windows redisplayed.
Don't update frame titles and menu bars for frames that don't need to
be redisplayed.
(propagate_buffer_redisplay): New function.
(AINC): New macro.
(redisplay_internal): Use it. Be more selective in the set of windows
we redisplay. Propagate windows_or_buffers_changed to
update_mode_lines a bit later to simplify the code.
(mark_window_display_accurate_1): Reset window and buffer's
`redisplay' flag.
(redisplay_window): Do nothing if neither the window nor the buffer nor
the frame needs redisplay.
* src/window.h (struct window): Add `redisplay' field.
(wset_redisplay, fset_redisplay, bset_redisplay, bset_update_mode_line)
(redisplay_other_windows, window_list): New declarations.
* src/window.c (select_window, Fset_window_start): Use wset_redisplay.
(window_list): Not static any more.
(grow_mini_window, shrink_mini_window): Use fset_redisplay.
* src/minibuf.c (read_minibuf_unwind): Don't redisplay everything.
* src/insdel.c (prepare_to_modify_buffer_1): Use bset_redisplay.
* src/frame.c (Fmake_frame_visible): Don't redisplay everything.
* src/frame.h (struct frame): Add `redisplay' field.
Move `external_menu_bar' bitfield next to other bit-fields.
(SET_FRAME_GARBAGED): Use fset_redisplay.
(SET_FRAME_VISIBLE): Don't garbage the frame;
Use redisplay_other_windows.
* src/buffer.h (struct buffer): Add `redisplay' field.
* src/buffer.c (Fforce_mode_line_update): Pay attention to the `all' flag.
(modify_overlay): Use bset_redisplay.
* src/alloc.c (gc_sweep): Don't unmark strings while sweeping symbols.
* lisp/doc-view.el (doc-view-goto-page): Update mode-line.
In GNU Emacs 24.3.50.1 (x86_64-apple-darwin13.0.0, NS apple-appkit-1265.00)
of 2013-12-13 on macpro.lan
Bzr revision: 115272 monnier@iro.umontreal.ca-20131128224309-jg2ar5frhpri4yow
Windowing system distributor `Apple', version 10.3.1265
Configured using:
`configure --with-ns'
Important settings:
value of $LC_CTYPE: UTF-8
locale-coding-system: utf-8-unix
default enable-multibyte-characters: t
Major mode: Lisp Interaction
Minor modes in effect:
follow-mode: t
tooltip-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 input:
<up> <up> <up> <up> <up> <up> <up> <up> <up> <up> <up>
<up> <down> <down> <down> <down> <down> <down> <down>
<down> <down> <down> <down> <down> <down> <down> <down>
C-x 3 <up> <up> <up> <up> <up> <up> <up> <up> <escape>
x f o l l o w - d e <backspace> <backspace> m o d e
<return> <up> <up> <up> <up> <up> <up> <up> <up> <up>
<up> <down> <down> <down> <down> <down> <down> <down>
<down> <down> <down> <down> <down> <down> <down> <down>
<down> <down> <down> <down> <down> <down> <down> <down>
<down> <down> <down> <down> <up> <up> <up> <up> <up>
<up> <up> <up> <up> <up> C-h v e m a c s - b z <tab>
<return> <up> <up> <up> <up> <up> <up> <up> <up> <down>
<down> <down> <down> <down> <down> <down> <down> <down>
<down> <down> <down> <down> <down> <down> <down> <down>
<down> <up> <up> <up> <up> <up> <up> C-x o C-x b *
s c <tab> <return> <up> <up> <up> <up> <up> <up> <up>
<up> <up> <up> <up> <up> <up> <up> <down> <down> C-x
1 <down> <down> <down> <down> <down> <down> <down>
<down> <down> <menu-bar> <help-menu> <send-emacs-bug-report>
F o l l o w - m o d e SPC s <backspace> i s SPC <backspace>
<backspace> <backspace> <backspace> <backspace> <backspace>
<backspace> <backspace> <backspace> <backspace> <backspace>
<backspace> <backspace> <backspace> <backspace> R e
d i d <backspace> <backspace> <backspace> <backspace>
<backspace> E m a c s SPC i s SPC s l o w SPC w h e
n SPC f o l l o w - m o d e SPC i s SPC a <backspace>
e n a b l e d SPC a n d SPC b u f f e r SPC t a i l
C-a <right> <right> <right> <right> <right> <right>
<right> <right> <right> <s-backspace> <menu-bar> <help-menu>
<send-emacs-bug-report>
Recent messages:
Beginning of buffer
End of buffer
Follow mode enabled
Beginning of buffer [4 times]
End of buffer [13 times]
Type "q" in help window to restore its previous buffer.
Beginning of buffer [4 times]
End of buffer [4 times]
Beginning of buffer [6 times]
<s-backspace> is undefined
Quit
Load-path shadows:
None found.
Features:
(shadow sort gnus-util mail-extr emacsbug message format-spec rfc822 mml
mml-sec mm-decode mm-bodies mm-encode mail-parse rfc2231 mailabbrev
gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums mm-util
mail-prsvr mail-utils pp help-mode help-fns follow easymenu time-date
tooltip ediff-hook vc-hooks lisp-float-type mwheel ns-win 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
cocoa ns multi-tty emacs)