Thanks for your insights, Eli!  Some follow-up thoughts & questions.

Eli Zaretskii <eliz@gnu.org> writes:

>> On my system, I happen to have
>> 
>> - one font (KpMath) that provides U+1D49C–U+1D4B5 (the "capital" chars),
>>   but not U+1D4B6–U+1D4CF (the "small" chars),
>> 
>> - another font (DejaVu Math TeX Gyre) that provides both sub-ranges[1].
>> 
>> As luck would have it, when trying to display a char in this range,
>> Emacs stumbles on KpMath first
>
> It's not Emacs that stumbles on that font, it's your font backend
> (probably fontconfig?).  Emacs just verifies that the candidate fonts
> returned by the backend satisfy the requirements, and uses the first
> font that does.  So this might mean a bug report for fontconfig and/or
> the font designers is in order.

Right; "as luck would have it", fontconfig returns KpMath rather than
DejaVu Math TeX Gyre; that's because Emacs only asks for U+1D49C, if I'm
reading ftfont.c's use of Vscript_representative_chars correctly?

So I'm not sure there's a bug on fontconfig's end?  FWIW the CLI tools
seem to provide accurate information[1].

>> E.g. in fontset.el, would it be overkill to break down the 52-char
>> ranges in the math-subgroup loop into 2 26-char ranges in order to
>> always add the "SMALL A" codepoints to script-representative-chars?
>
> What would be the purpose of breaking that range?
> script-representative-chars can specify multiple characters per
> script, see its doc string.

Right now script-representative-chars contains this:

> (mathematical-script 119964)

With this patch: