bug#27008: 26.0.50; auto-hscroll-mode and scroll-left

[Stephen Berman <stephen.berman@gmx.net>]

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When auto-hscroll-mode is set to `current-line' and scroll-left is
invoked with arguments ARG > 0 and SET-MINIMUM non-nil, then when the
current line is automatically horizontally scrolled, all other lines in
the buffer are scrolled back to logical BOL, i.e. SET-MINIMUM is ignored
(except on the current line).  To reproduce:

0. emacs -Q
1. Set auto-hscroll-mode to `current-line'.
2. Type `C-x C-f /path/to/hscroll-bug RET' (the attached file).
3. Type `M-x toggle-truncate-lines' and `M-: (scroll-left 32 t)'.
4. Type `C-p' repeatedly.
=> When point is on the third line, and for all subsequent vertical
   motion, all lines but the current one are displayed starting at BOL
   instead of column SET-MINIMUM.

(I found this bug because I sometimes use scroll-left with non-nil
SET-MINIMUM in the Gnus *Summary* buffer to see more of the article
Subject lines; the attached file hscroll-bug, used in the above recipe,
is taken from such a buffer.)

In GNU Emacs 26.0.50 (build 14, x86_64-pc-linux-gnu, GTK+ Version 3.22.8)
 of 2017-05-21 built on rosalinde
Repository revision: 08212929ba7052883bd506be320dfaaae5b68970
Windowing system distributor 'The X.Org Foundation', version 11.0.11901000

Configured using:
 'configure 'CFLAGS=-Og -g3''

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

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


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   (  56) Eli Zaretskii        │ master c189986: Add an optional arguments to string-trim
   (  74) Eli Zaretskii        │ master d35da21: Improve documentation of 'split-string'
   (  12) Michael Albinus      │ ▶ master updated (d35da21 -> 1b0ec9f)
   (  30) Michael Albinus      │ master 6de77cf 1/2: Fix a problem with OpenSSH 7 in Tramp
   (  56) Michael Albinus      │ master 1b0ec9f 2/2: Minor tweaks in tramp-tests.el
   (  85) Eli Zaretskii        │ master c1c6f16: Fix turning off whitespace-mode
   (  30) Eli Zaretskii        │ emacs-25 e80f6a2: Describe problems with Microsoft Intellipoint
   ( 117) Stefan Monnier       │ master b372e56: * lisp/emacs-lisp/package.el: Quote `package-desc' in docstrings
   (  24) Eli Zaretskii        │ master c7391db: ; * doc/emacs/files.texi (Auto Save Files): Fix a cross-reference.
   (  65) Paul Eggert          │ master 7ff8c5c: Minor .gitignore fixes
   (  35) Paul Eggert          │ master c1c8b67: Check that signed right shift is arithmetic
   (  16) Noam Postavsky       │ ▶ master updated (c1c8b67 -> acd58c9)
   ( 284) Noam Postavsky       │ master 267be4b 1/2: Refactor lisp eval result printing
   ( 379) Noam Postavsky       │ master acd58c9 2/2: Limit integers printed as characters (Bug#16828)
   (  27) Paul Eggert          │ master a5acb37: Port --enable-gcc-warnings to clang 3.9.1
   (  34) Paul Eggert          │ master 7d00410: Add handlerlist assertion to module code
   ( 234) Eli Zaretskii        │ master 1cbbece: Fix automatic hscrolling of only the current line
   ( 138) Eli Zaretskii        │ master 021430f: New commands: find-library-other-window, find-library-other-frame
   (  33) Eli Zaretskii        │ master 6c7bf03: Avoid crashes in GC due to unescaped characters warning
   ( 656) Philipp Stephani     │ master 31fded0: Reimplement module functions