bug#19993: 25.0.50; Unicode fonts defective on Windows

[Ilya Zakharevich]

On Sun, Mar 08, 2015 at 12:38:05AM -0800, Ilya Zakharevich wrote:
> When Emacs wants to show a character using a fontset:
>   • Emacs looks in the fontset and finds the font specifications associated
>     to this character.
>   • Emacs checks which Unicode Subset contains the given character.
>               (What if not unique???)
>   • From fonts matching the font specifications, Emacs picks up those
>     which have this Unicode Subset “identified” within the font.
>   • From these, Emacs choses one (which?).
> 
> Emacs uses this procedure for two fontsets: the currently enabled one, and
> the default fontset.  If none of two obtained fonts supports the given
> character, a HEX representation is shown.
> =======================================================

Interesting tidbit:

  57    D800 - DFFF     Non-Plane 0. Note that setting this bit implies that
                        there is at least one supplementary code point
                        beyond the Basic Multilingual Plane (BMP) that
                        is supported by this font. See Surrogates and
                        Supplementary Characters.

Extrapolating (since there is no other way to treat this), having a
Subset “identified” may mean just that there is at least 1 character
in this range supported by the font.  ;-)  :-(

Ilya