As @simoncozens and @madig noted in googlefonts/ufo2ft#635, in order to make sure the variable-first kernFeatureWriter produces the same result (when compiled to GPOS) as the multiple-masters-then-varlib-merged one, the kerning dictionaries for each masters need to be "unioned" or "aligned" to properly take care of differing kerning "exceptions" in the various masters. In other words, kerns missing from some masters need to be "backfilled", not just with zeros but using the more "general" value available for the given glyph pair in each master (falling back to zeros if none), see:

googlefonts/ufo2ft#635 (comment)
googlefonts/ufo2ft#635 (comment)

Nikolaus has written a script to do that on a designspace+UFOs, see https://gist.github.com/madig/76567a9650de639bbff51ce010783790

We should incorporate this logic in when we convert kerning data from glyphs/UFO => IR. We cannot leave unspecified those missing kerning pairs, because our variable-first IR model for kerning (map of {KernPair: {Location: float}}) would then incorrectly imply that, if a pair doesn't specify a value for a given location, it should be interpolated at that location ("sparse kerning"), but the current font sources do not work like that. Master kerning dictionaries are not intended to be "sparse" in our current font source formats.

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rsheeteropenedon May 26, 2023

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rsheeter
openedon May 26, 2023