Created attachment 8610060 [details] Mixed GNU Free Font, Latin Modern Math and Asana Math This issue was raised during discussion with the author of GNU Free Fonts. If you open http://fred-wang.github.io/MathFonts/GNUFreeSerif/ and scroll down to, for example, the sigma summation then you see that other math fonts than GNU Free fonts may used (Latin Modern and Asana), while the document only specifies font-family = GNU Free font. I think it's fine to use glyphs from the fallback fonts when the document font is not able to stretch the desired operator, just like what happens with "normal" characters missing from one font. However, when the document font does have something to stretch a given operator we should probably try and use the best size from that font and not a better size from another font. I believe this is related to what was discussed in bug 584332 comment 10. I have not read the code again, but I suspect this is not restricted to large operators, though. Besides providing more consistent rendering of a document with respect to what the author requests, this could help for bug 1009582 since we would stop at the first MATH font that has an entry for the desired operator, instead of trying all the fonts in the list.
Created attachment 8610061 [details] Screenshot with fallback fonts disabled This is a screenshot of the same document, with the preferences options for fallback fonts cleared in about:config. Only the sizes provided by GNU Free Fonts are used.
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