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

From: clemera <[email-masked]>
Date: Thu, 9 May 2019 21:15:26 +0200

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:
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Configured features:
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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:
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  global-eldoc-mode: t
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  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:
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rfc2045 ietf-drums mm-util mail-prsvr mail-utils elec-pair time-date
mule-util tooltip eldoc electric uniquify ediff-hook vc-hooks
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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)
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 (vectors 16 14656)
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