bug#26605: 25.1; emacs picks unsuitable fallback fonts for monospace default font

bug#26605: 25.1; emacs picks unsuitable fallback fonts for monospace default font

[Ralf Jung]

Hi all,

I have my system set up to use "Fira Mono" as the default font for
"Monospace", and my code editors (emacs and Kate) use that font as their
default font.  However, if I use emacs without any further
configuration, it picks unsuitable fallback fonts for characters not
supported my Fira Mono.  "Unsuitable" here means in particular that the
chosen fallback font is not monospace, so lines that contain fallback
characters break the grid.  At the same time, I do have suitable fonts
installed, as witnessed by Kate being able to preserve the grid.

Concretely, consider the following example text (pieced together from a
codebase using unicode characters for mathematical operators):

  mixin_cmra_pcore_mono x y cx :
    x ≼ y → pcore x = Some cx → ∃ cy, pcore y = Some cy ∧ cx ≼ cy;
    ✓{n} x → x ≡{n}≡ y1 ⋅ y2 →
    z1 z2, x ≡ z1 ⋅ z2 ∧ z1 ≡{n}≡ y1 ∧ z2 ≡{n}≡ y2
    ∀ ✓{n} (∅:A).
    }.

(At least) the following characters are not supported by Fira Mono, so a
fallback is picked:
  ≼ ✓ ⋅ ∧ ∅ ∃ ∀

As a result, the above text does not stay within the "monotext grid",
some characters are significantly wider than the majority (e.g., ≼)
while others are much smaller (⋅).  However, all of these characters are
supported by DejaVu Mono, a font I have installed.  I believe that's the
font other editors are using to render these characters.  When I
configure emacs to use DejaVu Mono for characters not supported by Fira
Mono, the text above looks entirely as expected.

Notice that, for example, → is supported by both Fira Mono and DejaVu
Mono.  In these cases, the editor should use the preferred font, which
is Fira Mono in my case.  (Actually, the much prettier arrows are one of
the reasons I prefer Fira Mono over DejaVu Mono.)

Ideally, emacs' default configuration would be such that it prefers
monospace over non-monospace fonts when falling back from a monospace font.

Kind regards,
Ralf

In GNU Emacs 25.1.1 (x86_64-pc-linux-gnu, GTK+ Version 3.22.5)
 of 2017-01-01, modified by Debian built on binet
Windowing system distributor 'The X.Org Foundation', version 11.0.11902000
System Description:	Debian GNU/Linux 9.0 (stretch)

Configured using:
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 --sharedstatedir=/var/lib --libexecdir=/usr/lib
 --localstatedir=/var/lib --infodir=/usr/share/info
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 --with-sound=alsa --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/emacs25:/etc/emacs:/usr/local/share/emacs/25.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/25.1/site-lisp:/usr/share/emacs/site-lisp
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-fstack-protector-strong
 -Wformat -Werror=format-security -Wall' 'CPPFLAGS=-Wdate-time
 -D_FORTIFY_SOURCE=2' LDFLAGS=-Wl,-z,relro'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GCONF GSETTINGS
NOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11

Important settings:
  value of $LC_MESSAGES: en_US.utf8
  value of $LC_NUMERIC: en_US.utf8
  value of $LANG: de_DE.utf8
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Major mode: Fundamental

Minor modes in effect:
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Recent messages:
Loading /etc/emacs/site-start.d/50autoconf.el (source)...done
Loading /etc/emacs/site-start.d/50cmake-data.el (source)...done
Loading /etc/emacs/site-start.d/50dictionaries-common.el (source)...
Loading debian-ispell...
Loading /var/cache/dictionaries-common/emacsen-ispell-default.el
(source)...done
Loading debian-ispell...done
Loading /var/cache/dictionaries-common/emacsen-ispell-dicts.el
(source)...done
Loading /etc/emacs/site-start.d/50dictionaries-common.el (source)...done
For information about GNU Emacs and the GNU system, type C-h C-a.
funcall-interactively: Buffer is read-only: #<buffer *GNU Emacs*>

Load-path shadows:
/usr/share/emacs/25.1/site-lisp/debian-startup hides
/usr/share/emacs/site-lisp/debian-startup
/usr/share/emacs25/site-lisp/cmake-data/cmake-mode hides
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bug#26605: 25.1; emacs picks unsuitable fallback fonts for monospace default font

[Eli Zaretskii]

> From: Ralf Jung <post@ralfj.de>
> Date: Sat, 22 Apr 2017 15:06:49 +0200
> 
> Notice that, for example, → is supported by both Fira Mono and DejaVu
> Mono.  In these cases, the editor should use the preferred font, which
> is Fira Mono in my case.

Isn't that what happens?  It does here.

bug#26605: 25.1; emacs picks unsuitable fallback fonts for monospace default font

[Ralf Jung]

Hi,

>> Notice that, for example, → is supported by both Fira Mono and DejaVu
>> Mono.  In these cases, the editor should use the preferred font, which
>> is Fira Mono in my case.
> 
> Isn't that what happens?  It does here.

Yes, sorry if that was unclear.  With the default setup, → is picking
the font I expected it to pick.
I mentioned this just because when changing the configuration to use
DejaVu Mono as a fallback font, it often happened to me that then emacs
also used DejaVu Mono for →.

Kind regards,
Ralf

bug#26605: 25.1; emacs picks unsuitable fallback fonts for monospace default font

[Eli Zaretskii]

> Cc: 26605@debbugs.gnu.org
> From: Ralf Jung <post@ralfj.de>
> Date: Sat, 22 Apr 2017 18:24:59 +0200
> 
> >> Notice that, for example, → is supported by both Fira Mono and DejaVu
> >> Mono.  In these cases, the editor should use the preferred font, which
> >> is Fira Mono in my case.
> > 
> > Isn't that what happens?  It does here.
> 
> Yes, sorry if that was unclear.  With the default setup, → is picking
> the font I expected it to pick.
> I mentioned this just because when changing the configuration to use
> DejaVu Mono as a fallback font, it often happened to me that then emacs
> also used DejaVu Mono for →.

Shouldn't happen in Emacs 25, AFAIK.  We specifically made a change in
that version to stick to the default font for symbols and punctuation
characters, as long as the default font supports them.  Maybe your
experience comes from older versions?

bug#26605: 25.1; emacs picks unsuitable fallback fonts for monospace default font

[Ralf Jung]

Hi,

> Shouldn't happen in Emacs 25, AFAIK.  We specifically made a change in
> that version to stick to the default font for symbols and punctuation
> characters, as long as the default font supports them.  Maybe your
> experience comes from older versions?

Fair enough, I tested this before upgrading.
So just ignore this part for now, the bug is about the default behavior
only.

; Ralf

bug#26605: 25.1; emacs picks unsuitable fallback fonts for monospace default font

[Lars Ingebrigtsen]

Ralf Jung <post@ralfj.de> writes:

> Ideally, emacs' default configuration would be such that it prefers
> monospace over non-monospace fonts when falling back from a monospace font.

That does sound reasonable.  Has Emacs grown such a feature in the years
since this bug was reported?

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

bug#26605: 25.1; emacs picks unsuitable fallback fonts for monospace default font

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Sun, 17 Nov 2019 08:50:53 +0100
> Cc: 26605@debbugs.gnu.org
> 
> Ralf Jung <post@ralfj.de> writes:
> 
> > Ideally, emacs' default configuration would be such that it prefers
> > monospace over non-monospace fonts when falling back from a monospace font.
> 
> That does sound reasonable.  Has Emacs grown such a feature in the years
> since this bug was reported?

No, I don't think so.  The fallback uses the fontsets, so defining
those to put monospaced fonts before variable-pitch ones is the way to
go.  AFAIK, there's no logic in Emacs to select fallback fonts without
using the fontsets (the default ones if the user didn't customize
them).