When passing a parsed feature file that has variable anchors to
addOpenTypeFeatures(), builder would modify the anchors in place and
discard the variations, which break any subsequent use of the feature
file.
I encountered this building a font that has variable cursing anchors
with ufo2ft. The cursFeatureWriter would write the variable anchors, but
then when kernFeatreWriter compiles the file to get GSUB closure, the
variation would be dropped from the anchors, and later when when the
feature data is compiled into the font, the anchors would be compiled
without variations.
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