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