I recently filed a bug report on the XeTeX repo about the kerning of sub- and superscripts in OpenType fonts:

    https://sourceforge.net/p/xetex/bugs/175/

luaTeX's find_math_kern() has many of the same flaws.

Cheers,
David.

P.S. I don't think the related bug

    https://sourceforge.net/p/xetex/bugs/173/

is relevant here because luaTeX's analyze_script() already does essentially the same thing (I haven't compared the two implementations in detail).  In fact, analyze_script() does more, since it also handles the sub_box case.

David Jones

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