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Recompute OS/2.usMaxContext when adding feature variations #3894

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Merged
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Aug 11, 2025
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Recompute OS/2.usMaxContext when adding feature variations #3894

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8 changes: 8 additions & 0 deletions Lib/fontTools/varLib/featureVars.py
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Original file line number Diff line number Diff line change
Expand Up @@ -95,6 +95,14 @@ def addFeatureVariations(font, conditionalSubstitutions, featureTag="rvrn"):

addFeatureVariationsRaw(font, font["GSUB"].table, conditionsAndLookups, featureTags)

# Update OS/2.usMaxContext in case the font didn't have features before, but
# does now, if the OS/2 table exists. The table may be required, but
# fontTools needs to be able to deal with non-standard fonts. Since feature
# variations are always 1:1 mappings, we can set the value to at least 1
# instead of recomputing it with `otlLib.maxContextCalc.maxCtxFont()`.
if (os2 := font.get("OS/2")) is not None:
os2.usMaxContext = max(1, os2.usMaxContext)


def _existingVariableFeatures(table):
existingFeatureVarsTags = set()
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4 changes: 4 additions & 0 deletions Tests/varLib/featureVars_test.py
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Expand Up @@ -21,6 +21,8 @@ def makeVariableFont(glyphOrder, axes):
axis.minimum, axis.default, axis.maximum = minimum, default, maximum
ds_axes[axisTag] = axis
varLib._add_fvar(font, ds_axes, instances=())
os2 = font["OS/2"] = newTable("OS/2")
os2.usMaxContext = 0
return font


Expand All @@ -35,7 +37,9 @@ def varfont():
def test_addFeatureVariations(varfont):
assert "GSUB" not in varfont

assert varfont["OS/2"].usMaxContext == 0
addFeatureVariations(varfont, [([{"wght": (0.5, 1.0)}], {"A": "A.alt"})])
assert varfont["OS/2"].usMaxContext == 1

assert "GSUB" in varfont
gsub = varfont["GSUB"].table
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