min/max width and stretchy operators

[fred-wang]

Just converting content from the OpenType appendix:

The OpenType MATH specification does not seem to take into account linebreaking. As explained in section 3.1.2 it is important in a HTML5 context to be able to determine the min-content width and the max-content width. However when an operator is stretched vertically to cover a target size, it is not possible to know the selected size variant or glyph assembly without knowing the target size and so the min-content and max-content widths can only be approximated. In practice, the width of the vertical operators is almost independent on its stretch size. Should that be a requirement of the OpenType MATH specification?

[fred-wang]

I think math font designers really want to have different width for different sizes, so it probably does not make sense to have such a restriction. Instead we should ensure to probably handle this min/max calculation in the spec. We currently do that, the only remaining thing I'd like to fix is w3c/mathml-core#123