OpenType MATH MathItalicCorrection

[fred-wang]

It is used during the layout of mrow/subsup/munderover

https://mathml-refresh.github.io/mathml-core/#layout-of-mrow
https://mathml-refresh.github.io/mathml-core/#subscripts-and-superscripts-msub-msup-msubsup
https://mathml-refresh.github.io/mathml-core/#underscripts-and-overscripts-munder-mover-munderover
https://mathml-refresh.github.io/mathml-core/#prescripts-and-tensor-indices-mmultiscripts

We have tests for that, but we probably need more.

[NSoiffer]

@fred-wang: can this be closed?

[fred-wang]

Let's keep it open, I need to check how much we implemented it in chromium and whether test coverage should be improved.