script-representative-chars would contain:

> (mathematical-script . [119964 119990])

I meant that suggestion as a band-aid fix if we deem that Emacs should
guard against these "incomplete" fonts.

Not sure it makes sense as-is though, since it yields a vector; judging
by script-representative-chars's docstring (and a cursory glance at
ftfont_list and font_match_p), a list would be more appropriate, to make
all codepoints mandatory?

>>     1. (push #x1D4CF (alist-get 'mathematical-script
>>                                 script-representative-chars))
>>     2. (set-fontset-font
>>         t 'mathematical "DejaVu Math TeX Gyre" nil 'prepend)
>>     3. Uninstall KpMath.
>
> I'd suggest to do 3 regardless.  A font that supports only part of a
> block is a bad font to have.

(Right; AFAICT KpMath got pulled in when I installed TeXLive.  I'm not
entirely sure why the font was designed this way; I couldn't find a
rationale in the documentation hosted on CTAN[2])

> Emacs should probably do the equivalent of 1

OK; I'll try to follow-up with a bug report and a patch SomeDay™ then.


Again, thanks for your explanations.


[1]
$ fc-list ':charset=1d4c1'
⇒ a bunch of fonts, including DejaVu Math TeX Gyre but *not* KpMath

$ fc-match -f '%{charset}\n' 'KpMath'
… 1d456-1d49c 1d49e-1d49f 1d4a2 1d4a5-1d4a6 1d4a9-1d4ac 1d4ae-1d4b5
1d4d0-1d4e9 …
⇒ includes U+1D49C–U+1D4B5 (minus nonexistent codepoints) but not
  U+1D4B6–U+1D4CF

$ fc-match -f '%{charset}\n' 'DejaVu Math TeX Gyre'
… 1d456-1d49c 1d49e-1d49f 1d4a2 1d4a5-1d4a6 1d4a9-1d4ac 1d4ae-1d4b9
1d4bb 1d4bd-1d4c3 1d4c5-1d505…
⇒ includes U+1D49C–U+1D4B5 *and* U+1D4B6–U+1D4CF (minus nonexistent
  codepoints)

[2]
https://ctan.org/pkg/kpfonts
https://ctan.org/pkg/kpfonts-otf

The "List of glyphs" document from this second page, § 15.1.8, features
a table that clearly shows that KpMath (column "K") does not contain the
lowercase glyphs, but I fail to find a design rationale.