bug#35659: 26.1; Weird horizontal scroll behavior with certain settings for `auto-hscroll-mode' and `hscroll-step`.

[clemera]

Hello,

After starting emacs -Q I set the following settings by evaluating them in 
*scratch*:

(setq auto-hscroll-mode 'current-line
      hscroll-step 1
      truncate-lines t)

Now after adding a long line right after the above code block (which exceeds 
the frame width)
go to the end of the long line. The line scrolls as expected but when you go 
from there to the lines above, those get
pushed to the left. I have recorded a gif which shows the described behavior:

https://imgur.com/a/lJYo4Od


In GNU Emacs 26.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.30)
 of 2019-01-03 built on clemera
Repository revision: ab525dd6b1c57cee7f26700f9656bb6c76a8ceba
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
System Description:     Ubuntu 18.04.2 LTS

Recent messages:
Making completion list...
nil
t
Mark set [17 times]
funcall-interactively: End of buffer [7 times]
Quit
user-error: Beginning of history; no preceding item
Quit [3 times]
nil
t
Quit
Configured using:
 'configure --with-xft --with-x-toolkit=gtk3 --with-modules'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS NOTIFY
GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB TOOLKIT_SCROLL_BARS
GTK3 X11 MODULES THREADS LCMS2

Important settings:
  value of $LC_MONETARY: de_DE.UTF-8
  value of $LC_NUMERIC: de_DE.UTF-8
  value of $LC_TIME: de_DE.UTF-8
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

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

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny seq byte-opt gv
bytecomp byte-compile cconv cl-loaddefs cl-lib dired dired-loaddefs
format-spec rfc822 mml easymenu mml-sec password-cache epa derived epg
epg-config gnus-util rmail rmail-loaddefs mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util mail-prsvr mail-utils elec-pair time-date
mule-util 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 menu-bar
rfn-eshadow isearch timer select scroll-bar mouse jit-lock font-lock
syntax facemenu font-core term/tty-colors frame 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 minibuffer
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 dbusbind inotify lcms2
dynamic-setting system-font-setting font-render-setting move-toolbar gtk
x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 98229 13360)
 (symbols 48 20364 1)
 (miscs 40 78 244)
 (strings 32 28371 1265)
 (string-bytes 1 747151)
 (vectors 16 14656)
 (vector-slots 8 497062 7570)
 (floats 8 50 298)
 (intervals 56 551 9)
 (buffers 992 12))