Make hb_ot_math_get_constant work around Cambria Math's bug for displayOperatorMinHeight/delimitedSubFormulaMinHeight

[fred-wang]

cc @khaledhosny

OpenType doc: https://learn.microsoft.com/en-us/typography/opentype/spec/math#mathconstants-table
Microsoft bug: MicrosoftDocs/typography-issues#1136 (comment)

Basically, Microsoft Word has a bug and reads displayOperatorMinHeight and delimitedSubFormulaMinHeight in the wrong order. Cambria Math seems to have the parameters inverted too, so that it still works with Microsoft Word. I don't know if because of that other math fonts were badly designed too, although I believed I reported bugs to font creators in the past.

I cannot say much about delimitedSubFormulaMinHeight. I'm not sure I understand where it's used for layout. Neither MathML Core not MathML implementations in browsers use it.

However, displayOperatorMinHeight is used by several MathML and TeX implementations to draw n-ary operators in display mode, and due to the Cambria Math bug these implementations generally ended up implementing a hack like "ensure the displaystyle is large enough, independently of the value of displayOperatorMinHeight". MathML Core and Chromium's implementation do not have such a hack.

So the question is: should HarfBuzz's hb_ot_math_get_constant work around Cambria Math's bug for displayOperatorMinHeight/delimitedSubFormulaMinHeight by inverting the value?

[behdad]

SGTM.

[fred-wang]

I'm not sure whether Microsoft plans to release a new version of Cambria Math with that bug fixed, or if other math fonts have the issue (would be interesting to check the parameters). Anyway, I'll give it a try.