bug#24169: 25.1.50; Line numbers disappear when horizontally scrolled

[ivan <ivan.brennan@gmail.com>]

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Using linum-mode displays line numbers to the left of the left fringe. I
would expect them to remain visible regardless of any horizontal scrolling,
since that's how the left fringe behaves (not to mention numerous other
editors and applications). However, when using line-truncation, the line
numbers disappear when the text is horizontally scrolled. I think this is
worth fixing, because it would be more useful and look better (the dead
space takes up screen real-estate and makes me sad).

To reproduce, run 'emacs -Q', then:

  1. Type a long line of text (enough to exhibit line-wrapping).

  2. Go to the beginning of line.
     C-a

  3. Turn on line truncation.
     M-: (setq truncate-lines t)

  4. Turn on line-numbers.
     M-x linum-mode

  5. Move cursor forward past the right-edge of the window.
     C-f ...

  6. When the beginning of line scrolls out of view, the line numbers
disappear.

  7. Move cursor back towards the beginning of line.
     C-b ...

  8. Confirm that the line-numbers reappear as soon as the beginning of
line is back in view.


System and version info below:

In GNU Emacs 25.1.50.1 (x86_64-apple-darwin15.6.0, NS appkit-1404.47
Version 10.11.6 (Build 15G31))
of 2016-07-28 built on MachineCode.local
Repository revision: ec359399a47f852b4d022a30245449438e349193
Windowing system distributor 'Apple', version 10.3.1404
Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
 'configure --disable-dependency-tracking --disable-silent-rules
 --enable-locallisppath=/usr/local/share/emacs/site-lisp
 --infodir=/usr/local/Cellar/emacs-plus/HEAD-ec35939/share/info/emacs
 --prefix=/usr/local/Cellar/emacs-plus/HEAD-ec35939 --with-xml2
 --without-dbus --with-gnutls --with-rsvg --with-imagemagick --with-ns
 --disable-ns-self-contained'

Configured features:
JPEG RSVG IMAGEMAGICK NOTIFY ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS
NS

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

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-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 subr-x puny seq byte-opt gv
bytecomp byte-compile cl-extra help-mode cconv cl-loaddefs pcase 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 time-date mule-util tooltip eldoc electric uniquify
ediff-hook vc-hooks lisp-float-type mwheel term/ns-win ns-win
ucs-normalize term/common-win tool-bar dnd fontset image regexp-opt
fringe tabulated-list newcomment elisp-mode lisp-mode prog-mode register
page menu-bar rfn-eshadow 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 charscript 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 kqueue cocoa ns multi-tty
make-network-process emacs)

Memory information:
((conses 16 205473 6422)
 (symbols 48 20545 0)
 (miscs 40 43 141)
 (strings 32 18220 6425)
 (string-bytes 1 585506)
 (vectors 16 34498)
 (vector-slots 8 666572 7372)
 (floats 8 177 13)
 (intervals 56 222 0)
 (buffers 976 11))

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