Hello,

This issue is a regression from emacs 25.x.

I discovered this issue while using multiple-cursors package that uses a
dynamic minor mode lighter that shows the active number of multiple cursors.

(defcustom mc/mode-line
  `(" mc:" (:eval (format ,(propertize "%d" 'face 'font-lock-warning-face)
                          (mc/num-cursors))))
  "What to display in the mode line while multiple-cursors-mode is active."
  :group 'multiple-cursors)

Ref:
https://github.com/magnars/multiple-cursors.el/blob/10752700084595bb24712c27ba70a2326302e45b/multiple-cursors-core.el#L523-L527

But the issue can be easily recreated using this dummy code:

=====
(defvar test-cache 0)

(defconst test-format-mode-lighter
  `(" test:" (:eval (format ,(propertize "%d" 'face
'font-lock-warning-face) (test-incr)))))

(defun test-incr ()
  (setq test-cache (1+ test-cache))
  test-cache)

(define-minor-mode test-format-mode
  "Test that the dynamic minor mode lighter does not get truncated."
  :lighter test-format-mode-lighter
  (unless test-format-mode
    (setq test-cache 0)))
=====

1. Start emacs -Q (emacs 26+).
2. Evaluate the above dummy code snippet.
3. M-x test-format-mode

You will see this:
[image: image.png]

4. Now move the cursor around.. you will see the number after "test:"
increment.. once it reaches 9, the next increment would be 10.. but then
the numbers disappear altogether. You see this now:

[image: image.png]

Upon doing C-h v test-format-mode-lighter, I see:

test-format-mode-lighter’s value is
(" test:"
 (:eval
   (format
    #("%d" 0 2
      (face font-lock-warning-face))
    (test-incr))))


I don't understand the exact meaning of those numbers "0 2", but looks like
that limits the number of characters to be displayed after "test:"?

As a quick hack, I replace the "%d" in that variable with "%-3d".

Now the C-h v gives:

test-format-mode-lighter’s value is
(" test:"
 (:eval
   (format
    #("%-3d" 0 4
      (face font-lock-warning-face))
    (test-incr))))

So the number of character limit got raised because "%-3d" is literally 4
characters.

If I make that "%-10d".. the limit is literally 5 characters!!

test-format-mode-lighter’s value is
(" test:"
 (:eval
   (format
    #("%-10d" 0 5
      (face font-lock-warning-face))
    (test-incr))))

This strange truncation behavior did not exist on emacs 25+.

My guess is that the

In GNU Emacs 26.0.60 (build 3, x86_64-pc-linux-gnu, GTK+ Version 2.24.23)
 of 2017-09-20
Repository revision: f16a8d5dbd3bb8a319c951bdde9a6a75dbdb8c17
Windowing system distributor 'The X.Org Foundation', version 11.0.60900000
System Description: Red Hat Enterprise Linux Workstation release 6.6
(Santiago)

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
 'configure --with-modules
 --prefix=/home/kmodi/usr_local/apps/6/emacs/emacs-26
 '--program-transform-name=s/^ctags$/ctags_emacs/'
 'CPPFLAGS=-I/home/kmodi/usr_local/6/include -I/usr/include/freetype2
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 PKG_CONFIG_PATH=/home/kmodi/usr_local/6/lib/pkgconfig:/home/kmodi/usr_local/6/lib64/pkgconfig:/cad/adi/apps/gnu/linux/x86_64/6/lib/pkgconfig:/cad/adi/apps/gnu/linux/x86_64/6/lib64/pkgconfig:/usr/lib/pkgconfig:/usr/lib64/pkgconfig:/usr/share/pkgconfig:/lib/pkgconfig:/lib64/pkgconfig'

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ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK2 X11 MODULES

Important settings:
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=none
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Major mode: Lisp Interaction

Minor modes in effect:
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  global-eldoc-mode: t
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  electric-indent-mode: t
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  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
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Load-path shadows:
None found.

Features:
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ethiopic indian cyrillic chinese composite charscript charprop
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cl-preloaded nadvice loaddefs button faces cus-face macroexp files
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custom widget hashtable-print-readable backquote dbusbind inotify
dynamic-setting system-font-setting font-render-setting move-toolbar gtk
x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 97048 7622)
 (symbols 48 20814 1)
 (miscs 40 42 119)
 (strings 32 28795 1337)
 (string-bytes 1 765341)
 (vectors 16 14940)
 (vector-slots 8 508725 5443)
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 (intervals 56 245 0)
 (buffers 992 11)
 (heap 1024 21678 835))

-- 

Kaushal Modi