bug#44284: 27.1; with some Unicode font, scrolling upward with the mouse wheel actually scrolls downward when the cursor needs repositioning

[Vincent Lefevre]

Under Debian/unstable, create a file with

  printf "%d\n" `seq 1000` > file

then open it using GNU Emacs 27.1 with

  emacs -Q -fn misc-fixed-medium-r-semicondensed--13-120-75-75-c-60-iso10646-1 
file

Go to the end of the file with ESC > then scroll upward with the
mouse wheel (button 4) several times (but slowly). When the cursor
needs repositioning, this actually scrolls downward, probably
because Emacs tries to center the mouse cursor instead of letting
it at the bottom.

No such issue with the other way round, where the cursor remains
at the top.

Note: I have old GNU Emacs 27.0.91 snapshot versions still installed,
one compiled on 2020-05-14 without cairo, and I cannot reproduce the
bug with it (the cursor remains at the bottom), and one compiled on
2020-05-15 with Cairo (--with-cairo), and the bug occurs with it.


In GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.23, cairo 
version 1.16.0)
 of 2020-10-24, modified by Debian built on x86-conova-01
Windowing system distributor 'The X.Org Foundation', version 11.0.12009000
System Description: Debian GNU/Linux bullseye/sid

Recent messages:
Loading /etc/emacs/site-start.d/50psvn.el (source)...done
Loading /etc/emacs/site-start.d/50reposurgeon.el (source)...done
Loading /etc/emacs/site-start.d/50texlive-lang-english.el (source)...done
Loading /etc/emacs/site-start.d/50w3m-el.el (source)...done
Loading /etc/emacs/site-start.d/50why3.el (source)...done
Loading /home/vlefevre/share/emacs/site-lisp/mutteditor.el (source)...done
Loading time...done
For information about GNU Emacs and the GNU system, type C-h C-a.
Mark set
<mouse-4> is undefined [3 times]

Configured using:
 'configure --build x86_64-linux-gnu --prefix=/usr
 --sharedstatedir=/var/lib --libexecdir=/usr/lib
 --localstatedir=/var/lib --infodir=/usr/share/info
 --mandir=/usr/share/man --enable-libsystemd --with-pop=yes
 
--enable-locallisppath=/etc/emacs:/usr/local/share/emacs/27.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/27.1/site-lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --with-mailutils --build
 x86_64-linux-gnu --prefix=/usr --sharedstatedir=/var/lib
 --libexecdir=/usr/lib --localstatedir=/var/lib
 --infodir=/usr/share/info --mandir=/usr/share/man --enable-libsystemd
 --with-pop=yes
 
--enable-locallisppath=/etc/emacs:/usr/local/share/emacs/27.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/27.1/site-lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --with-mailutils --with-cairo
 --with-x=yes --with-x-toolkit=gtk3 --with-toolkit-scroll-bars
 'CFLAGS=-g -O2 -fdebug-prefix-map=/build/emacs-bB10hX/emacs-27.1+1=.
 -fstack-protector-strong -Wformat -Werror=format-security -Wall'
 'CPPFLAGS=-Wdate-time -D_FORTIFY_SOURCE=2' LDFLAGS=-Wl,-z,relro'

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

Important settings:
  value of $LC_COLLATE: POSIX
  value of $LC_CTYPE: C.UTF-8
  value of $LC_TIME: en_DK.utf8
  value of $LANG: POSIX
  locale-coding-system: utf-8-unix

Major mode: Fundamental

Minor modes in effect:
  display-time-mode: t
  show-paren-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-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
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
/usr/share/emacs/site-lisp/llvm-10/llvm-mode hides 
/usr/share/emacs/site-lisp/llvm-3.6/llvm-mode
/usr/share/emacs/site-lisp/llvm-10/tablegen-mode hides 
/usr/share/emacs/site-lisp/llvm-3.6/tablegen-mode
/usr/share/emacs/site-lisp/llvm-10/emacs hides 
/usr/share/emacs/site-lisp/llvm-3.6/emacs
/usr/share/emacs/site-lisp/llvm-10/llvm-mode hides 
/usr/share/emacs/site-lisp/llvm-3.7/llvm-mode
/usr/share/emacs/site-lisp/llvm-10/tablegen-mode hides 
/usr/share/emacs/site-lisp/llvm-3.7/tablegen-mode
/usr/share/emacs/site-lisp/llvm-10/emacs hides 
/usr/share/emacs/site-lisp/llvm-3.7/emacs
/usr/share/emacs/site-lisp/llvm-10/llvm-mode hides 
/usr/share/emacs/site-lisp/llvm-6.0/llvm-mode
/usr/share/emacs/site-lisp/llvm-10/tablegen-mode hides 
/usr/share/emacs/site-lisp/llvm-6.0/tablegen-mode
/usr/share/emacs/site-lisp/llvm-10/emacs hides 
/usr/share/emacs/site-lisp/llvm-6.0/emacs
/usr/share/emacs/site-lisp/llvm-10/llvm-mode hides 
/usr/share/emacs/site-lisp/llvm-7/llvm-mode
/usr/share/emacs/site-lisp/llvm-10/tablegen-mode hides 
/usr/share/emacs/site-lisp/llvm-7/tablegen-mode
/usr/share/emacs/site-lisp/llvm-10/emacs hides 
/usr/share/emacs/site-lisp/llvm-7/emacs
/usr/share/emacs/site-lisp/llvm-10/llvm-mode hides 
/usr/share/emacs/site-lisp/llvm-8/llvm-mode
/usr/share/emacs/site-lisp/llvm-10/tablegen-mode hides 
/usr/share/emacs/site-lisp/llvm-8/tablegen-mode
/usr/share/emacs/site-lisp/llvm-10/emacs hides 
/usr/share/emacs/site-lisp/llvm-8/emacs
/usr/share/emacs/site-lisp/llvm-10/llvm-mode hides 
/usr/share/emacs/site-lisp/llvm-9/llvm-mode
/usr/share/emacs/site-lisp/llvm-10/tablegen-mode hides 
/usr/share/emacs/site-lisp/llvm-9/tablegen-mode
/usr/share/emacs/site-lisp/llvm-10/emacs hides 
/usr/share/emacs/site-lisp/llvm-9/emacs
/usr/share/emacs/site-lisp/flim/md4 hides /usr/share/emacs/27.1/lisp/md4
/usr/share/emacs/site-lisp/flim/hex-util hides 
/usr/share/emacs/27.1/lisp/hex-util
/usr/share/emacs/site-lisp/flim/sasl-cram hides 
/usr/share/emacs/27.1/lisp/net/sasl-cram
/usr/share/emacs/site-lisp/flim/hmac-md5 hides 
/usr/share/emacs/27.1/lisp/net/hmac-md5
/usr/share/emacs/site-lisp/flim/hmac-def hides 
/usr/share/emacs/27.1/lisp/net/hmac-def
/usr/share/emacs/site-lisp/flim/sasl-digest hides 
/usr/share/emacs/27.1/lisp/net/sasl-digest
/usr/share/emacs/site-lisp/flim/sasl hides /usr/share/emacs/27.1/lisp/net/sasl
/usr/share/emacs/site-lisp/flim/sasl-ntlm hides 
/usr/share/emacs/27.1/lisp/net/sasl-ntlm
/usr/share/emacs/site-lisp/flim/ntlm hides /usr/share/emacs/27.1/lisp/net/ntlm
/usr/share/emacs/site-lisp/latex-cjk-thai/thai-word hides 
/usr/share/emacs/27.1/lisp/language/thai-word

Features:
(shadow sort mail-extr warnings 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 sendmail rfc2047 rfc2045 ietf-drums
mm-util mail-prsvr mail-utils time cus-start cus-load paren cc-styles
cc-align cc-engine cc-vars cc-defs edmacro kmacro cl-loaddefs cl-lib
w3m-load tooltip eldoc electric uniquify ediff-hook vc-hooks
lisp-float-type 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 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 dbusbind
inotify lcms2 dynamic-setting system-font-setting font-render-setting
cairo move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 70231 7098)
 (symbols 48 9062 1)
 (strings 32 22942 1331)
 (string-bytes 1 738076)
 (vectors 16 10797)
 (vector-slots 8 143077 9264)
 (floats 8 26 24)
 (intervals 56 252 0)
 (buffers 1000 13))