bug#59347: 29.0.50; `:family` face setting ignored

[Yuan Fu]

> On Nov 18, 2022, at 9:21 AM, Eli Zaretskii <eliz@gnu.org> wrote:
> 
>> Cc: 59347@debbugs.gnu.org
>> Date: Fri, 18 Nov 2022 18:54:02 +0200
>> From: Eli Zaretskii <eliz@gnu.org>
>> 
>>> What should I change in my recipe in order to keep the same default font
>>> but get the DejaVu Sans that used to get?
>> 
>> The default font would be my guess.  Try using some other font, not
>> from the fixed-misc family.
> 
> I take that back: I tried your recipe, and it works with every font I
> tried except DejaVu Sans.  So I guess that font is the culprit, and
> you should find some other font that you like.
> 
> Why DejaVu Sans is rejected, I donb't know, but that font has some
> issues that we already discovered in the past, so it could be a good
> idea to get rid of it regardless.

I might have some more information, and here is a reproduce of what I see: 

Load these two (open source) fonts in to your machine, and emacs -Q -l 
reproduce.el

The content of reproduce.el is:

(pop-to-buffer
 (let ((font1 "IBM Plex Mono")
       (font2 "Charter"))
   (with-current-buffer (get-buffer-create "*test*")
     (set-face-attribute 'default nil
                         :font (font-spec :family font1
                                          :weight 'medium))
     (insert (propertize "Some Text\n" 'face `(:family ,font2)))
     (insert (propertize "Some Text"
                         'face `(:font ,(font-spec :family font2))))
     (current-buffer))))

It inserts two lines of text, both using font2, but the first line using 
:family and the second using :font. The one using :family is not displayed in 
font2 (it falls back to some other font), but the one using :font is.

I think this is because the default font (font1) uses medium weight, but font2 
doesn’t have a medium weight. I tried with different fonts for font2, and 
whether that font has a medium weight correlates to whether the first line of 
text can be displayed in that font.

So my guess is that if the face uses the :family attribute, it inherits the 
weight from default, and if Emacs cannot find that weight in that font, it 
falls back to some other font.

I don’t know how to “fix” this, but at very least we should make it easy to 
figure out why the family attribute “didn’t work”. (It’s not unreasonable for 
someone to think: I have the font on my machine, the family settings is set to 
that font, why is the text not displayed in that font??)

Personally I think falling back to the same font but different weight is 
probably less confusing.

Yuan

reproduce.el
Description: Binary data

IBMPlexMono-Medium.ttf
Description: font/ttf

Charter Regular.otf
Description: font/otf