Subject: bug#66764: 29.1; Emacs scrolls for "(goto-char (point-max))" instead of jumping

From: Geza Herman <[email-masked]>
Date: Thu, 26 Oct 2023 19:04:42 +0200

The bug is, that in certain circumstances, emacs moves the point by 
using some kind of scrolling for "(goto-char (point-max))", instead of 
just jumping.

Repro:

- create a file using the little C program at the end (it produces a 
large text to stdout, you'll need to redirect it to some file)

- open the file with "emacs -Q generated.txt"

- with M-:, execute this:

    (progn
      (setq scroll-conservatively 101)
      (font-lock-add-keywords nil '(((lambda (bound)) (1 'error prepend 
t))) t)
      (goto-char (point-max)))

Notice that emacs doesn't jump to the end immediately as usual, but it 
does some kind of very fast scrolling.

(With long-line-threshold set to nil, the issue doesn't happen)

In my real case, a much smaller file produces this problem. Also, Emacs 
doesn't go to the end of the file, but stops somewhere in the middle (I 
was unable to reproduce this issue with a simple configuration). So to 
go to the end of the file I have to run "(goto-char (point-max))" 
multiple times. Interestingly, "M->" works fine. But if I remove the 
recenter call at the end of "end-of-buffer", it also has this odd behavior.

Here is the C program:

#include <stdio.h>
#include <stdlib.h>

int main() {
     for (int i=0; i<200000; i++) {
         int len = 100 + (rand()&0x7f);
         if (i==5000) len = 60000;
         for (int j=0; j<len; j++) {
             printf("x");
         }
         printf("\n");
     }
}

In GNU Emacs 29.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.38,
  cairo version 1.16.0) of 2023-08-30, modified by Debian built on
  x86-csail-01
Windowing system distributor 'The X.Org Foundation', version 11.0.12101008
System Description: Debian GNU/Linux trixie/sid

Configured using:
  'configure --build x86_64-linux-gnu --prefix=/usr
  --sharedstatedir=/var/lib --libexecdir=/usr/libexec
  --localstatedir=/var/lib --infodir=/usr/share/info
  --mandir=/usr/share/man --with-libsystemd --with-pop=yes
  --enable-locallisppath=/etc/emacs:/usr/local/share/emacs/29.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/29.1/site-lisp:/usr/share/emacs/site-lisp
  --with-sound=alsa --without-gconf --with-mailutils
  --with-native-compilation --build x86_64-linux-gnu --prefix=/usr
  --sharedstatedir=/var/lib --libexecdir=/usr/libexec
  --localstatedir=/var/lib --infodir=/usr/share/info
  --mandir=/usr/share/man --with-libsystemd --with-pop=yes
  --enable-locallisppath=/etc/emacs:/usr/local/share/emacs/29.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/29.1/site-lisp:/usr/share/emacs/site-lisp
  --with-sound=alsa --without-gconf --with-mailutils
  --with-native-compilation --with-cairo --with-x=yes
  --with-x-toolkit=gtk3 --with-toolkit-scroll-bars 'CFLAGS=-g -O2
  -ffile-prefix-map=/build/reproducible-path/emacs-29.1+1=. 
-fstack-protector-strong
  -Wformat -Werror=format-security -Wall' 'CPPFLAGS=-Wdate-time
  -D_FORTIFY_SOURCE=2' LDFLAGS=-Wl,-z,relro'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES
NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP SOUND SQLITE3
THREADS TIFF TOOLKIT_SCROLL_BARS TREE_SITTER WEBP X11 XDBE XIM XINPUT2
XPM GTK3 ZLIB

Important settings:
   value of $LC_ALL: C.UTF-8
   value of $LANG: en_US.UTF-8
   value of $XMODIFIERS: @im=ibus
   locale-coding-system: utf-8-unix

Major mode: Text

Minor modes in effect:
   tooltip-mode: t
   global-eldoc-mode: t
   show-paren-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
   line-number-mode: t
   indent-tabs-mode: t
   transient-mark-mode: t
   auto-composition-mode: t
   auto-encryption-mode: t
   auto-compression-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message mailcap yank-media puny dired
dired-loaddefs rfc822 mml mml-sec password-cache epa derived epg rfc6068
epg-config gnus-util text-property-search time-date mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail
rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils comp comp-cstr
warnings icons subr-x rx cl-seq cl-macs gv cl-extra help-mode
cl-loaddefs cl-lib bytecomp byte-compile rmc iso-transl tooltip cconv
eldoc paren electric uniquify ediff-hook vc-hooks lisp-float-type
elisp-mode mwheel term/x-win x-win term/common-win x-dnd tool-bar dnd
fontset image regexp-opt fringe tabulated-list replace newcomment
text-mode lisp-mode prog-mode register page tab-bar menu-bar rfn-eshadow
isearch easymenu timer select scroll-bar mouse jit-lock font-lock syntax
font-core term/tty-colors frame minibuffer nadvice seq simple cl-generic
indonesian philippine cham georgian utf-8-lang misc-lang vietnamese
tibetan thai tai-viet lao korean japanese eucjp-ms cp51932 hebrew greek
romanian slovak czech european ethiopic indian cyrillic chinese
composite emoji-zwj charscript charprop case-table epa-hook
jka-cmpr-hook help abbrev obarray oclosure cl-preloaded button loaddefs
theme-loaddefs faces cus-face macroexp files window text-properties
overlay sha1 md5 base64 format env code-pages mule custom widget keymap
hashtable-print-readable backquote threads dbusbind inotify lcms2
dynamic-setting system-font-setting font-render-setting cairo
move-toolbar gtk x-toolkit xinput2 x multi-tty make-network-process
native-compile emacs)

Memory information:
((conses 16 100665 12371)
  (symbols 48 7141 0)
  (strings 32 26857 2252)
  (string-bytes 1 845777)
  (vectors 16 20215)
  (vector-slots 8 367701 18494)
  (floats 8 50 17)
  (intervals 56 2211 0)
  (buffers 984 12))