bug#17892: 24.3; display error when mode-line-format is set to nil

bug#17892: 24.3; display error when mode-line-format is set to nil

[Joost Kremers]

This bug report will be sent to the Bug-GNU-Emacs mailing list
and the GNU bug tracker at debbugs.gnu.org.  Please check that
the From: line contains a valid email address.  After a delay of up
to one day, you should receive an acknowledgment at that address.

Please write in English if possible, as the Emacs maintainers
usually do not have translators for other languages.

Please describe exactly what actions triggered the bug, and
the precise symptoms of the bug.  If you can, give a recipe
starting from `emacs -Q':

====================

- Start Emacs with `emacs -Q -nw'
- load a (longish) text file (should be more than one screen full of text)
- (set-window-margins nil 40 40) ; use values suitable for the window
- (setq-local mode-line-format nil)

At this point, the text is centered on the screen, except for the last
line of the file (the one that is displayed where the mode line would
be). Instead of having a large left margin, it is displayed with a
margin of 0.

This only occurs when Emacs is run without GUI, regardless of whether
it's on a terminal emulator or a Linux console. It is apparently a
problem with the display engine, which assumes that there is always a
mode line present. Note, however, that setting `mode-line-format' to nil
is mentioned in the Elisp manual (info "(elisp) Mode Line Top") and in
the doc string of `mode-line-format' as a way to disable the mode line.

See also the thread on gnu.emacs.help, where I brought up the issue:

<http://lists.gnu.org/archive/html/help-gnu-emacs/2014-06/msg00463.html>

Thanks,

Joost Kremers

====================

If Emacs crashed, and you have the Emacs process in the gdb debugger,
please include the output from the following gdb commands:
    `bt full' and `xbacktrace'.
For information about debugging Emacs, please read the file
/usr/share/emacs/24.3/etc/DEBUG.

In GNU Emacs 24.3.1 (x86_64-pc-linux-gnu, GTK+ Version 3.4.2)
 of 2014-02-22 on king, modified by Debian
System Description:	elementary OS Luna

Configured using:
 `configure '--build' 'x86_64-linux-gnu' '--build' 'x86_64-linux-gnu'
 '--prefix=/usr' '--sharedstatedir=/var/lib' '--libexecdir=/usr/lib'
 '--localstatedir=/var/lib' '--infodir=/usr/share/info'
 '--mandir=/usr/share/man' '--with-pop=yes'
 '--enable-locallisppath=/etc/emacs24:/etc/emacs:/usr/local/share/emacs/24.3/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/24.3/site-lisp:/usr/share/emacs/site-lisp'
 '--with-crt-dir=/usr/lib/x86_64-linux-gnu' '--with-x=yes'
 '--with-x-toolkit=gtk3' '--with-toolkit-scroll-bars'
 'build_alias=x86_64-linux-gnu' 'CFLAGS=-g -O2 -fstack-protector
 --param=ssp-buffer-size=4 -Wformat -Wformat-security
 -Werror=format-security -Wall' 'LDFLAGS=-Wl,-Bsymbolic-functions
 -Wl,-z,relro' 'CPPFLAGS=-D_FORTIFY_SOURCE=2''

Important settings:
  value of $LC_MONETARY: en_IE.UTF-8
  value of $LC_NUMERIC: en_IE.UTF-8
  value of $LC_TIME: en_IE.UTF-8
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: Text

Minor modes in effect:
  tooltip-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
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent input:
C-x C-f e b TAB DEL DEL s r TAB e b TAB m a n TAB e 
b TAB t RET ESC : ( s e t - w i n d TAB m a r TAB SPC 
n i l SPC 4 0 SPC 4 0 ) RET ESC : ( s e t q - l o c 
a l SPC m o d e l - DEL DEL - l i n e - f o r m a t 
SPC n i l ) RET ESC x r e p o r t - e m a c s - b u 
g RET

Recent messages:
("emacs")
Loading term/xterm...done
For information about GNU Emacs and the GNU system, type C-h C-a.
Loading vc-git...done
nil [2 times]

Load-path shadows:
None found.

Features:
(shadow sort gnus-util mail-extr emacsbug message format-spec rfc822 mml
easymenu mml-sec mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums
mm-util mail-prsvr mail-utils vc-git time-date tooltip ediff-hook
vc-hooks lisp-float-type mwheel x-win x-dnd tool-bar dnd fontset image
regexp-opt fringe tabulated-list newcomment lisp-mode register page
menu-bar rfn-eshadow timer select scroll-bar mouse jit-lock font-lock
syntax facemenu font-core frame cham georgian utf-8-lang misc-lang
vietnamese tibetan thai tai-viet lao korean japanese hebrew greek
romanian slovak czech european ethiopic indian cyrillic chinese
case-table epa-hook jka-cmpr-hook help simple abbrev minibuffer loaddefs
button faces cus-face macroexp files text-properties overlay sha1 md5
base64 format env code-pages mule custom widget hashtable-print-readable
backquote make-network-process dbusbind dynamic-setting
system-font-setting font-render-setting move-toolbar gtk x-toolkit x
multi-tty emacs)



-- 
Joost Kremers
Life has its moments

bug#17892: 24.3; display error when mode-line-format is set to nil

[Eli Zaretskii]

> From: Joost Kremers <joostkremers@fastmail.fm>
> Date: Tue, 01 Jul 2014 10:17:03 +0200
> 
> - Start Emacs with `emacs -Q -nw'
> - load a (longish) text file (should be more than one screen full of text)
> - (set-window-margins nil 40 40) ; use values suitable for the window
> - (setq-local mode-line-format nil)
> 
> At this point, the text is centered on the screen, except for the last
> line of the file (the one that is displayed where the mode line would
> be). Instead of having a large left margin, it is displayed with a
> margin of 0.

For the record, this bug was there since Emacs 21.

> This only occurs when Emacs is run without GUI, regardless of whether
> it's on a terminal emulator or a Linux console.

No, the bug was present in the GUI display as well, but it was almost
impossible to see it on GUI frames.  That's because GUI frames
allocate their glyph matrices with dimensions larger than what you
normally see, to be able to display the smallest frame's font, if and
when it is used in some face.  So the glyph row where this bug was
present is almost never used for displaying normal text in the extra
line vacated by the mode line whose format is nil.

By contrast, text-mode frames allocate the glyph matrices of precisely
the dimensions they need, so when the mode line is switched off, its
glyph row is always reused for displaying the extra line of text.

It seems like a rainy day today, so I fixed this bug in revision
117336 on the emacs-24 branch.