Emacs 28: Specific TTF font gets loaded with font-backend x instead of ftcrhb

[Tassilo Horn]

Hi all,

I use the current Emacs from the master branch.  It is configured with
cairo and HarfBuzz support.

  (frame-parameter nil 'font-backend) ;=> (ftcrhb x)

I don't configure fonts inside emacs but it takes my default monospaced
font from GNOME settings.  Right now, that is set to PragmataPro, so
describe-char says something like

    display: by this font (glyph code)
    ftcrhb:-FSD -PragmataPro-normal-normal-normal-*-15-*-*-*-*-0-iso10646-1 
(#x46)

So far, so good.  When I now switch my GNOME monospace font setting to
the JetBrains Mono font [1] and fire up a new frame, it seems like a
bitmap variant of that font is used.  describe-char says:

    display: by this font (glyph code)
    x:-misc-jetbrains mono-medium-r-normal--15-108-100-100-m-90-iso8859-1 (#x6F)

So indeed, here the font-backend is x rather than ftcrhb.  But both
PragmataPro and JetBrainsMono are normal TTF fonts.  fc-list knows them
both:

--8<---------------cut here---------------start------------->8---
➜ fc-list|egrep '(Jet|Pragmata)'|sort
/usr/share/fonts/TTF/FSD - PragmataPro Bold Italic.ttf: PragmataPro:style=Bold 
Italic
/usr/share/fonts/TTF/FSD - PragmataPro Bold.ttf: PragmataPro:style=Bold
/usr/share/fonts/TTF/FSD - PragmataPro Italic.ttf: PragmataPro:style=Italic
/usr/share/fonts/TTF/FSD - PragmataPro Liga Bold Italic.ttf: PragmataPro 
Liga:style=Bold Italic
/usr/share/fonts/TTF/FSD - PragmataPro Liga Bold.ttf: PragmataPro 
Liga:style=Bold
/usr/share/fonts/TTF/FSD - PragmataPro Liga Italic.ttf: PragmataPro 
Liga:style=Italic
/usr/share/fonts/TTF/FSD - PragmataPro Liga Regular.ttf: PragmataPro 
Liga:style=Regular
/usr/share/fonts/TTF/FSD - PragmataPro Mono Bold Italic.ttf: PragmataPro 
Mono:style=Bold Italic
/usr/share/fonts/TTF/FSD - PragmataPro Mono Bold.ttf: PragmataPro 
Mono:style=Bold
/usr/share/fonts/TTF/FSD - PragmataPro Mono Italic.ttf: PragmataPro 
Mono:style=Italic
/usr/share/fonts/TTF/FSD - PragmataPro Mono Liga Bold Italic.ttf: PragmataPro 
Mono Liga:style=Bold Italic
/usr/share/fonts/TTF/FSD - PragmataPro Mono Liga Bold.ttf: PragmataPro Mono 
Liga:style=Bold
/usr/share/fonts/TTF/FSD - PragmataPro Mono Liga Italic.ttf: PragmataPro Mono 
Liga:style=Italic
/usr/share/fonts/TTF/FSD - PragmataPro Mono Liga Regular.ttf: PragmataPro Mono 
Liga:style=Regular
/usr/share/fonts/TTF/FSD - PragmataPro Mono Regular.ttf: PragmataPro 
Mono:style=Regular
/usr/share/fonts/TTF/FSD - PragmataPro Regular.ttf: PragmataPro:style=Regular
/usr/share/fonts/TTF/JetBrainsMono-Bold-Italic.ttf: JetBrains Mono:style=Bold 
Italic
/usr/share/fonts/TTF/JetBrainsMono-Bold.ttf: JetBrains Mono:style=Bold
/usr/share/fonts/TTF/JetBrainsMono-ExtraBold-Italic.ttf: JetBrains 
Mono,JetBrains Mono ExtraBold:style=Extra Bold Italic,Italic
/usr/share/fonts/TTF/JetBrainsMono-ExtraBold.ttf: JetBrains Mono,JetBrains Mono 
ExtraBold:style=Extra Bold,Regular
/usr/share/fonts/TTF/JetBrainsMono-Italic.ttf: JetBrains Mono:style=Italic
/usr/share/fonts/TTF/JetBrainsMono-Medium-Italic.ttf: JetBrains Mono,JetBrains 
Mono Medium:style=Medium Italic,Italic
/usr/share/fonts/TTF/JetBrainsMono-Medium.ttf: JetBrains Mono,JetBrains Mono 
Medium:style=Medium,Regular
/usr/share/fonts/TTF/JetBrainsMono-Regular.ttf: JetBrains Mono:style=Regular
--8<---------------cut here---------------end--------------->8---

I've tried other fonts as well (like DejaVu Mono) and they all work just
fine in Emacs.  It's just that single one which doesn't work in emacs
but it does work in other apps (like all GNOME apps).

Any idea how to debug what's the problem with that font?

Thanks,
  Tassilo

[1] https://github.com/JetBrains/JetBrainsMono