bug#38407: 27.0.50; infinite loop with display of large file without newlines

[Pieter van Oostrum <pieter@vanoostrum.org>]

1) Load a large file, that contains no newlines. I used a 100KB json
file from my Firefox profile (but loaded as a text file)
2) set visual-line-mode aka Word wrap
3) scroll up (<next> scroll-up-command)

The scrolling is very slow, and the further in the file you come, the
slower it becomes. During that time, Emacs consumes close to 100% CPU.
At a certain moment it took me half an our for one scroll operation.
When I went to the end of the buffer (M-> end-of-buffer), Emacs looped
virtually forever. I decided to kill it after 12 hours.

In another experiment, I repeatedly did a scroll-up-command, followed by
the shell command 'date >> display.log'. I repeated this with a keyboard
macro.
The written times indicate that in the beginning the time for a
scroll-up goes up linearly until more than a minute, and then Emacs get
stuck.




In GNU Emacs 27.0.50 (build 1, i686-apple-darwin10.0.0, NS appkit-1561.61 Version 10.13.6 (Build 17G9016))
 of 2019-11-27 built on cochabamba.vanoostrum.org
Repository revision: 4eb7db5d4b84708912c63a77569c8adeeff6c640
Repository branch: master
Windowing system distributor 'Apple', version 10.3.1561
System Description:  Mac OS X 10.13.6

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
user-error: Beginning of history; no preceding item
 [Beginning of history; no preceding item]
Visual-Line mode enabled

Configured using:
 'configure --build i686-apple-darwin10.0.0 --without-dbus --with-ns
 build_alias=i686-apple-darwin10.0.0 'CFLAGS=-pipe -march=nocona'
 PKG_CONFIG_PATH=/usr/X11R6/pkgconfig/:/usr/local/lib/pkgconfig/:/usr/lib/pkgconfig/'

Configured features:
NOTIFY KQUEUE ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS MODULES
THREADS PDUMPER LCMS2

Important settings:
  value of $LC_CTYPE: UTF-8
  value of $LANG: en_NL.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Text

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-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
  visual-line-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
format-spec rfc822 mml easymenu mml-sec password-cache epa derived epg
epg-config gnus-util rmail rmail-loaddefs text-property-search time-date
subr-x seq byte-opt gv bytecomp byte-compile cconv mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader cl-loaddefs
cl-lib sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel term/ns-win ns-win ucs-normalize mule-util term/common-win
tool-bar dnd fontset image regexp-opt fringe tabulated-list replace
newcomment text-mode elisp-mode lisp-mode prog-mode register page
tab-bar menu-bar rfn-eshadow isearch timer select scroll-bar mouse
jit-lock font-lock syntax facemenu font-core term/tty-colors frame
minibuffer cl-generic 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 charscript charprop case-table epa-hook jka-cmpr-hook help
simple abbrev obarray cl-preloaded 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
threads kqueue cocoa ns lcms2 multi-tty make-network-process emacs)

Memory information:
((conses 16 47303 8427)
 (symbols 48 6678 1)
 (strings 32 16380 1383)
 (string-bytes 1 518147)
 (vectors 16 10228)
 (vector-slots 8 141557 8968)
 (floats 8 20 39)
 (intervals 56 193 0)
 (buffers 1000 13))

-- 
Pieter van Oostrum <pieter@vanoostrum.org>
www: http://pieter.vanoostrum.org/
PGP key: [8DAE142BE17999C4]