bug#31717: 26.1; display-line-numbers-mode enlarges indicator without need

[Carlos Pita <carlosjosepita@gmail.com>]

When transitioning from line 69 to line 70 a new column is added to the
left of the indicator as if there was a need to fit three digits despite
that the file has less than 100 lines and despite that if it were larger
line 100 would fall out of the window.

I have tested the transition from 969 to 970 and it shows the same
effect.

It's as if the mode is anticipating an extra digit soon and playing
conservative. I understand that there could be performance reasons for
doing this (thus avoiding the need to compute the remaining lines in a
window or file) but:

1. The mode is already adding two whitespaces around the line number
indicator. Wasting one more char is on the verge of infuriating ;)

2. AFAICS computing the file size doesn't seem too demanding a task,
neither the remaining lines in a window  (although the window could be
resized).

---

In GNU Emacs 26.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.30)
 of 2018-05-29 built on juergen
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
System Description:	Manjaro Linux

Recent messages:
Quit [2 times]
C-h C-g is undefined
nil
Quit
Display-Line-Numbers mode disabled in current buffer
x is undefined
Display-Line-Numbers mode enabled in current buffer
0 (#o0, #x0, ?\C-@)
scroll-down-command: Beginning of buffer [3 times]
Making completion list...

Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --with-x-toolkit=gtk3 --with-xft --with-modules
 'CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong
 -fno-plt' CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now'

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 MODULES THREADS LIBSYSTEMD LCMS2

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

Major mode: Info

Minor modes in effect:
  display-line-numbers-mode: t
  diff-auto-refine-mode: t
  pyvenv-mode: t
  shell-dirtrack-mode: t
  ido-everywhere: t
  winner-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-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
  buffer-read-only: t
  column-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
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rfc822 mml mml-sec epa derived epg 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 pp cl-print jka-compr vc-git display-line-numbers
doom-tomorrow-night-theme doom-themes doom-themes-common cl-extra
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company edmacro kmacro pcase help-fns radix-tree help-mode elpy
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easy-mmode elpy-shell pyvenv esh-var esh-io esh-cmd esh-opt esh-ext
esh-proc esh-arg esh-groups eshell esh-module esh-mode esh-util
elpy-profile elpy-django s elpy-refactor subr-x python tramp-sh tramp
tramp-compat tramp-loaddefs trampver ucs-normalize shell pcomplete
parse-time format-spec advice json map ido grep compile comint
ansi-color files-x etags xref project cus-edit cus-start cus-load
wid-edit winner ring windmove finder-inf info package easymenu
epg-config url-handlers url-parse auth-source cl-seq eieio eieio-core
cl-macs eieio-loaddefs password-cache url-vars seq byte-opt gv bytecomp
byte-compile cconv cl-loaddefs cl-lib 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
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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
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hebrew greek romanian slovak czech european ethiopic indian cyrillic
chinese composite charscript charprop case-table epa-hook jka-cmpr-hook
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make-network-process emacs)

Memory information:
((conses 16 395972 45995)
 (symbols 48 33197 1)
 (miscs 40 181 236)
 (strings 32 77765 6307)
 (string-bytes 1 2252569)
 (vectors 16 53016)
 (vector-slots 8 923948 18050)
 (floats 8 186 268)
 (intervals 56 4758 0)
 (buffers 992 16))