[HarfBuzz] 'vert' substitutions in CJK fonts

[Grigori Goronzy]

Hi,

a user of my library reported that vertical alternates are not correctly
subtituted in many CJK fonts. I am a bit puzzled by this.

When doing vertical layout of Japanese text, the 'vert' feature is
enabled in the library to select vertical variants of some Kanji, Kana
and punctuation characters. This works fine with many fonts, but with
some it does not.

The GSUB table of problematic fonts typically looks a bit too...
minimal. Here's an example, that's the "MOTOYA LMaru" font, Android's
standard CJK font:

>   Table  0 of 16: GSUB (0x0000010c+0x0000016c)
>     1 script(s) found in table
>     Script  0 of  1: kana
>       No default language system
>       1 language system(s) found in script
>       Language System  0 of  1: JAN 
>         No required feature
>         1 feature(s) found in language system
>         Feature index  0 of  1: 0
>     1 feature(s) found in table
>     Feature  0 of  1: vert; 1 lookup(s)
>         1 lookup(s) found in feature
>         Lookup index  0 of  1: 0
>     1 lookup(s) found in table
>     Lookup  0 of  1: type 1, props 0x0001

So subtitutions are only used if the run that is shaped has Katakana
(kana) script and language is set to Japanese (JAN). It works if I
explicitly set the language and force the script to Katakana.

But, in practice, that's of course not true! First, it breaks as soon as
the system language is not Japanese, unless the language has been
overridden. Second, not only Katakana characters have vertical variants.
Punctuation might or might not be substituted depending on context,
because punctuation characters have common script and assume the script
of characters around them. If they're next to Kanji characters, it will
break.

Should fonts with GSUB tables like that considered broken? What does
Uniscribe do to make this work? And lastly, can I force HarfBuzz to just
use the first 'vert' substitution lookup in case there's none to be
found with matching or DFLT script/language system?

Best regards
Grigori