[Bug 2013083] New: 31-cantarell.conf causes inconsistent font choice for numerals and symbols when the system locale is zh-*

https://bugzilla.redhat.com/show_bug.cgi?id=2013083

            Bug ID: 2013083
           Summary: 31-cantarell.conf causes inconsistent font choice for
                    numerals and symbols when the system locale is zh-*
           Product: Fedora
           Version: 35
            Status: NEW
         Component: abattis-cantarell-fonts
          Assignee: klember(a)redhat.com
          Reporter: vtq-gnome(a)outlook.com
        QA Contact: extras-qa(a)fedoraproject.org
                CC: cosimo.cecchi(a)gmail.com,
                    fonts-bugs(a)lists.fedoraproject.org,
                    klember(a)redhat.com, mark(a)net-c.com, me(a)fale.io,
                    tagoh(a)redhat.com
  Target Milestone: ---
           Link ID: GNOME Gitlab GNOME/gnome-shell/-/issues/4683
    Classification: Fedora



Created attachment 1832054
  --> https://bugzilla.redhat.com/attachment.cgi?id=1832054&action=edit
Inconsistent font for numerals and symbols in Nautilus

Description of problem:

In Fedora Workstation, the GNOME desktop is using Cantarell as its main font.
However, the following section in /etc/fonts/conf.d/31-cantarell.conf, which
comes from Fedora's abattis-cantarell-fonts package, allows the font choice to
be overridden:

  <match target="pattern">
    <test qual="any" name="family">
      <string>Cantarell</string>
    </test>
    <edit name="family" mode="assign" binding="weak">
      <string>Cantarell</string>
    </edit>
  </match>

$ env LANG=en_US.utf8 fc-match Cantarell
Cantarell-Regular.otf: "Cantarell" "Regular"
$ env LANG=zh_CN.utf8 fc-match Cantarell
NotoSansCJK-Regular.ttc: "Noto Sans CJK SC" "Regular"

GTK/Pango seems to select a font based on the language of the text, and the
decision is affected by the context. When the text is only numerals and symbols
it will assume the language from the locale. This kind of decisions seems to be
reasonable too. So for example, Latin script will still be displayed with
Cantarell. "1234567890" will be displayed with Noto Sans CJK while
"1234567890
GNOME" will be displayed with Cantarell (see screenshot):
$ env LANG=zh_CN.utf8 pango-view --markup -t '<span face="Cantarell"
font_features="tnum" size="xx-large">1234567890</span>'
$ env LANG=zh_CN.utf8 pango-view --markup -t '<span face="Cantarell"
font_features="tnum" size="xx-large">1234567890
GNOME</span>'

As a result, numerals and symbols in the GNOME UI are sometimes displayed with
Cantarell and sometimes displayed with Noto Sans CJK (see screenshots). The
inconsistency makes the UI appear broken. This is especially noticeable for
time in various places of the UI, because time is displayed in Noto Sans CJK
and the separator between hour and minute (a ratio symbol, which is a
full-width glyth in Noto Sans CJK and many other CJK fonts) has extra blank
space around it. (See https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/4683)


Version-Release number of selected component (if applicable):

0.301-3.fc35


How reproducible:

Always


Steps to Reproduce:

1. In GNOME desktop, choose "中文 臺灣", "中文 香港", or "汉语 中国" in
"Settings" –
"Region & Language" – "Language".
2. Log out & log in.


Actual results:

Numerals and symbols in the GNOME UI are sometimes displayed with Cantarell and
sometimes displayed with Noto Sans CJK (see screenshots).


Expected results:

Numerals and symbols should be consistently displayed with the same font,
ideally the same as the Latin alphabet. Since GNOME explicitly requests
Cantarell as UI font, Cantarell should be used here. Noto Sans CJK would only
be used as fallback for Chinese characters which are not covered by Cantarell.

Note that other common Latin fonts in the system are not overridden by Noto
Sans CJK like Cantarell is:
$ env LANG=zh_CN.utf8 fc-match "DejaVu Sans"
DejaVuSans.ttf: "DejaVu Sans" "Regular"
$ env LANG=zh_CN.utf8 fc-match "Liberation Sans"
LiberationSans-Regular.ttf: "Liberation Sans" "Regular"
$ env LANG=zh_CN.utf8 fc-match "Nimbus Sans"
NimbusSans-Regular.otf: "Nimbus Sans" "Regular"

Generally in mixed use of Latin and Chinese fonts, it is common to use the
selected Latin font for numbers and symbols and only fallback to the Chinese
font for glyths not covered. This is seen in word processors (e.g.
LibreOffice), Flatpak apps (because the Freedesktop runtime doesn't have font
config similar to the 31-cantarell.conf), as well as other OS (e.g. MacOS,
Android).

By deleting the previously mentioned fontconfig section in 31-cantarell.conf,
the expected results can be achieved (see screenshots).

env LANG=zh_CN.utf8 fc-match Cantarell
Cantarell-Regular.otf: "Cantarell" "Regular"


Additional info:

In my test without the relevant fontconfig section everything seems to be
working very well and I've not noticed any adverse effect. But I'm not aware of
why it was originally included in the package. 

An alternative solution is to use Noto Sans CJK for everything if the locale is
zh-*. But this is less favorable because it's against upstream choice of
Cantarell and needs a change in gsettings-desktop-schemas for these locales.
The clock separator issue would additionally need workarounds in Nautilus and
in GNOME translations to fix.


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https://bugzilla.redhat.com/show_bug.cgi?id=2013083