bug#58538: anti-alias fix results in ugly scaled fonts

[Eli Zaretskii <eliz@gnu.org>]

> From: Stephen Gildea <stepheng+emacs@gildea.com>
> Date: Fri, 14 Oct 2022 21:07:23 -0700
> 
> At HEAD, I get an ugly scaled font for my sans-serif text.
> I noticed it in MH-E, and you can reproduce it as follows:
> 
>     emacs -Q -fn fixed --eval "(buffer-face-set 'variable-pitch)"
> 
> Although the default font is specified on the command line as "fixed"
> (from the misc-fixed X11 family of fonts), and the variable-pitch font
> selected is not in the misc-fixed family, having "fixed" as the
> default font affects the font selected for the "variable-pitch" face.
> Which font it selects changed recently.
> 
> This bug seems to have been introduced by commit 6b1ed2f2c9,
> "Fix antialias face attribute when text is scaled", 27 August.
> 
> In an Emacs built before that commit, the above command displays text
> with a pretty sans-serif font.

The above works fine on my system: I get a "fixed" font without the
"--eval" argument, and variable-pitch font with it, as expected.

It sounds like so do you, but you don't like the variable-pitch font
you get, is that so?  If so, then the problem is with the fonts you
have installed, not necessarily with what Emacs produces.  That is,
assuming you do get variable-pitch font in the buffer, why are you
saying that Emacs doesn't perform correctly?  The font being "ugly" is
something only you know.

I'm guessing that what you see is due to some attributes of the
"fixed" font used as the default face's font.  The commit you
identified as the culprit preserves the attributes of the original
font which the face doesn't override, instead of throwing them away.
So I think the pretty sans-serif font you wanted to see is rejected
because it doesn't fit some of those attributes of the default face's
font.  Can you try figuring out which attribute(s) are those?  It
could be the :weight attribute, for example (the "medium" vs "regular"
issue, which is new in Emacs 29), or maybe :size.

> That commit undoes some of the changes that addressed bug#17973.
> Bug#17973 fixed display of misc-fixed fonts.  With that recent commit,
> using misc-fixed fonts again breaks some faces.

Please tell the details: which faces are broken and how?