bug#47498: 28.0.50; follow-mode misses a line with tiling window manager

bug#47498: 28.0.50; follow-mode misses a line with tiling window manager

[Stefan Kangas]

I see one hidden line when using follow-mode.

Steps to reproduce:

0. emacs -Q
1. C-h i
2. C-x 2
3. M-x follow-mode RET

There is now one line missing from the display.  When using
`display-line-numbers-mode', this effect is more obvious.

I use the tiling window manager XMonad, and Emacs is forced to full
screen.  It has the same dimensions as my screen, and I have no window
decorations.  My resolution is 2560x1440 and the `default' face is:

ftcrhb:-Bits-Bitstream Vera Sans
Mono-normal-normal-normal-*-16-*-*-*-m-0-iso10646-1

My guess is that the last line is displayed, but only the top few pixels
are visible, so it looks like it is not there at all.

When I change the Emacs window to be floating and resize manually, I
can't reproduce this bug.  The window size will then snap to lines and
all lines are visible.

This bug is reproducible with both Lucid and GTK.


In GNU Emacs 28.0.50 (build 8, x86_64-pc-linux-gnu, X toolkit, cairo
version 1.16.0, Xaw3d scroll bars)
 of 2021-03-29 built on joffe
Repository revision: 67ef8c36bde32a8bf33b118aa38f8339b0183c46
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12010000
System Description: Debian GNU/Linux bullseye/sid

Configured using:
 'configure --with-x-toolkit=lucid
 PKG_CONFIG_PATH=/home/skangas/usr/lib/pkgconfig:/home/skangas/usr/lib/pkgconfig:'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES NOTIFY
INOTIFY PDUMPER PNG RSVG SOUND THREADS TIFF TOOLKIT_SCROLL_BARS X11
XAW3D XDBE XIM XPM LUCID ZLIB

Important settings:
  value of $LC_COLLATE: C
  value of $LC_CTYPE: sv_SE.UTF-8
  value of $LC_TIME: C
  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
  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 rmc puny dired dired-loaddefs
rfc822 mml mml-sec epa derived epg epg-config gnus-util rmail
rmail-loaddefs auth-source cl-seq eieio eieio-core cl-macs
eieio-loaddefs password-cache json map text-property-search time-date
subr-x seq byte-opt gv bytecomp byte-compile cconv mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader cl-loaddefs
cl-lib sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
iso-transl 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 tabulated-list replace newcomment
text-mode elisp-mode lisp-mode prog-mode register page tab-bar menu-bar
rfn-eshadow isearch easymenu timer select scroll-bar mouse jit-lock
font-lock syntax font-core term/tty-colors frame minibuffer 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 composite charscript charprop
case-table epa-hook jka-cmpr-hook help simple abbrev obarray
cl-preloaded nadvice button loaddefs faces cus-face macroexp files
window text-properties overlay sha1 md5 base64 format env code-pages
mule custom widget hashtable-print-readable backquote threads dbusbind
inotify lcms2 dynamic-setting system-font-setting font-render-setting
cairo x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 50799 8163)
 (symbols 48 6514 1)
 (strings 32 18738 1753)
 (string-bytes 1 606639)
 (vectors 16 13181)
 (vector-slots 8 174052 8138)
 (floats 8 23 39)
 (intervals 56 227 0)
 (buffers 992 10))

bug#47498: 28.0.50; follow-mode misses a line with tiling window manager

[Eli Zaretskii]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Tue, 30 Mar 2021 12:30:42 +0000
> 
> I see one hidden line when using follow-mode.
> 
> Steps to reproduce:
> 
> 0. emacs -Q
> 1. C-h i
> 2. C-x 2
> 3. M-x follow-mode RET
> 
> There is now one line missing from the display.  When using
> `display-line-numbers-mode', this effect is more obvious.

From my POV, follow-mode's design is fundamentally flawed, when
variable-size fonts and fractional line display are considered.  It
attempts to solve several problems that cannot be solved in principle,
unless the only font on display is the default face's font, and the
windows display an integral number of text lines.  It's a small wonder
in my eyes that display problems caused by follow-mode are so minor,
even subtle.  My advice is to stay away of it, if you want perfect
display.

bug#47498: 28.0.50; follow-mode misses a line with tiling window manager

[Eli Zaretskii]

> Date: Tue, 30 Mar 2021 15:56:00 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 47498@debbugs.gnu.org
> 
> From my POV, follow-mode's design is fundamentally flawed, when
> variable-size fonts and fractional line display are considered.

Oh, and let's not forget features like display properties and
overlays, which can disrupt the display continuity if they happen on
the last line of the left window and continue into the first line of
the right one.  That's because follow-mode works by forcing the
window's start point, but Emacs is unable to have the window-start
point in the middle of a display property or overlay string.

bug#47498: 28.0.50; follow-mode misses a line with tiling window manager

[martin rudalics]

 > 0. emacs -Q
 > 1. C-h i
 > 2. C-x 2

Why C-x 2?  The whole idea of `follow-mode' is based on C-x 3.  Or am I
missing something?

 > 3. M-x follow-mode RET
 >
 > There is now one line missing from the display.  When using
 > `display-line-numbers-mode', this effect is more obvious.

Which line?  Can you post a screenshot?

martin

bug#47498: 28.0.50; follow-mode misses a line with tiling window manager

[Stefan Kangas]

[-- Attachment #1: Type: text/plain, Size: 466 bytes --]

martin rudalics <rudalics@gmx.at> writes:

>  > 0. emacs -Q
>  > 1. C-h i
>  > 2. C-x 2
>
> Why C-x 2?  The whole idea of `follow-mode' is based on C-x 3.  Or am I
> missing something?

Sorry, yes, that step should indeed be `C-x 3'.

>  > 3. M-x follow-mode RET
>  >
>  > There is now one line missing from the display.  When using
>  > `display-line-numbers-mode', this effect is more obvious.
>
> Which line?  Can you post a screenshot?

Please see the attached.

[-- Attachment #2: bug-47498.png --]
[-- Type: image/png, Size: 368744 bytes --]

bug#47498: 28.0.50; follow-mode misses a line with tiling window manager

[Eli Zaretskii]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Tue, 30 Mar 2021 10:36:23 -0500
> 
> >  > 3. M-x follow-mode RET
> >  >
> >  > There is now one line missing from the display.  When using
> >  > `display-line-numbers-mode', this effect is more obvious.
> >
> > Which line?  Can you post a screenshot?
> 
> Please see the attached.

Probably because there's a header-line, or the bread-crumbs display
property at the top of the window.

bug#47498: 28.0.50; follow-mode misses a line with tiling window manager

[martin rudalics]

 >> Which line?  Can you post a screenshot?
 >
 > Please see the attached.

I see it now here.  Looks like a problem with `pos-visible-in-window-p'
when the frame is maximized.  How these could be related is not clear to
me.

martin

bug#47498: 28.0.50; follow-mode misses a line with tiling window manager

[Stefan Kangas]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Stefan Kangas <stefan@marxist.se>
>> Date: Tue, 30 Mar 2021 10:36:23 -0500
>>
>> >  > 3. M-x follow-mode RET
>> >  >
>> >  > There is now one line missing from the display.  When using
>> >  > `display-line-numbers-mode', this effect is more obvious.
>> >
>> > Which line?  Can you post a screenshot?
>>
>> Please see the attached.
>
> Probably because there's a header-line, or the bread-crumbs display
> property at the top of the window.

Yes, something like that.  I can't reproduce this in e.g. `C-h C-n'.