bug#20628: 25.0.50; Incorrect line height for some fonts

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 22 May 2015 15:03:24 -0400
> From: Clément Pit--Claudel
>  <clement.pitclaudel@live.com>
> CC: 20628@debbugs.gnu.org
> 
> I believe you have something like the following line in mind:
> 
>   (set-fontset-font fontset 'unicode (font-spec :name "Symbola") nil 'append)

That's too radical.  You could be more selective, e.g.:

  (set-fontset-font "fontset-default"
                    '(#x1d400 . #x1d7ff)
		    "Symbola")

That's because you may wish using other fonts for other Unicode
blocks.

> Indeed, this fixes the problem. Unfortunately, this problem makes it hard for package developers to make use of prettify-symbols-mode. Indeed, programming languages like Agda or Gallina (Coq) would gain a lot from heavy prettification, but since the default fallback font tends to be one of these badly behaved TeX fonts, users of Adga and Coq packages will often run into this problem if we enable prettification by default at the package level the package level. IOW, it's currently hard to come up with a workaround that does not involve user intervention at the moment.

Couldn't those package developers recommend fontset settings, of even
include ready-to-use .emacs snippets as part of the package?

> > (...)
> >> Is it possible for Emacs to do so by default?
> > 
> > You mean, have the default fontset set up to avoid the problem?  The
> > difficulty with that is that the fonts we'd need to put into the
> > default fontset are not free, and there's an understandable reluctance
> > to advertise them.
> 
> Symbola (in package ttf-ancient-fonts in Debian) actually seems to have good coverage for the kind of math symbols that triggers fallback to TeX fonts.

Yes, and there are others (I mentioned them in my other message).