bug#24560: 25.1; With `--font 7x13', wrong font is used for Unicode characters

[Eli Zaretskii <eliz@gnu.org>]

Ping!

> Date: Thu, 29 Sep 2016 18:30:37 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 24560@debbugs.gnu.org
> 
> Adding Handa-san to this discussion, in the hope that he could shed
> some light on this issue.
> 
> > From: Vladimir Panteleev <thecybershadow@gmail.com>
> > Cc: 24560@debbugs.gnu.org
> > Date: Wed, 28 Sep 2016 21:58:20 +0000
> > 
> > On 2016-09-28 20:16, Eli Zaretskii wrote:
> > > Are you sure the 7x13 font installed on your system supports all those
> > > characters?  Emacs won't switch to a different font if the default
> > > font supports the character it needs to display.
> > 
> > As I mentioned in my bug report and as is visible in the screenshots,
> > the characters are indeed present in the font and are rendered correctly
> > in other applications. Only Emacs exhibits this problem.
> > 
> > > Or could it be that some customizations, either yours or site-wide,
> > > force Emacs to use other fonts for the above characters?
> > 
> > This problem is reproducible with "emacs -Q", without any
> > customizations, on all distributions I've tried.
> 
> Then I guess there's some factor at work here that I don't quite
> figure out.  Maybe the Emacs font selection algorithm doesn't like
> that font for some reason?
> 
> If you start Emacs with 12x13 font or 8x13, does Emacs display more
> characters than 7x13 before it switches to another font?
> 
> > Here is a complete recipe using a live CD, to remove any possibility of
> > user or site customization:
> > 
> > 1. Get the Antergos live ISO (
> > https://antergos.com/download/antergos-live-iso/ ) and boot from it
> > (e.g. in a VM).
> > 2. Choose "Start Antergos live".
> > 3. Choose "Try It".
> > 4. Install Emacs and urxvt (in a terminal, run "pacman -S emacs
> > rxvt-unicode").
> > 5. Run "emacs -Q --font 7x13".
> > 6. M-x view-hello-file
> > 7. Observe that Cyrillic characters are rendered in a 12x13 font.
> > 8. Run "urxvt -fn 7x13".
> > 9. Copy some Cyrillic characters from Emacs and paste them in urxvt.
> > 10. Observe that urxvt renders the Cyrillic characters correctly, in the
> > correct font.
> > 
> > This is not specific to Arch / Antegros: The problem is also
> > reproducible on Ubuntu and Fedora Workstation Live CDs, though you may
> > have to also install some font packages (xorg-x11-fonts-misc for
> > Fedora), and Emacs uses the 8x13 font for Cyrillic characters (so the
> > problem is less obvious). urxvt still uses the correct font there.
> > 
> > >> I have found that evaluating the Sexpr:
> > >>
> > >> (set-fontset-font "fontset-default" 'unicode "-misc-fixed-*-*-*--13-*-*-*-*-70-iso10646-1")
> > >>
> > > 
> > > Don't do that, this setting will cause all kinds of surprises.  To
> > > customize the fonts, use specific character codepoint ranges, and only
> > > specify font for a range if the font support most or all of that
> > > range.
> > 
> > Noted. However, immediately lacking a better concrete workaround, I
> > don't have a better option at my disposal.
> 
> Hopefully, Handa-san will have some advice here.