Handle spacing marks in a variable way

Lib/ufo2ft/featureWriters/kernFeatureWriter.py

@@ -394,6 +394,9 @@ def _makePairPosRule(self, pair, side1Classes, side2Classes, rtl=False):
             enumerated=enumerated,
         )
 
+    def _filterSpacingMarks(self, marks):
+        return [mark for mark in marks if self.context.font[mark].width != 0]
+
     def _makeKerningLookup(self, name, ignoreMarks=True):
         lookup = ast.LookupBlock(name)
         if ignoreMarks and self.options.ignoreMarks:
@@ -404,7 +407,7 @@ def _makeKerningLookup(self, name, ignoreMarks=True):
 
             spacing = []
             if marks:
-                spacing = [mark for mark in marks if self.context.font[mark].width != 0]
+                spacing = self._filterSpacingMarks(marks)
             if not spacing:
                 # Simple case, there are no spacing ("Spacing Combining") marks,
                 # do what we've always done.

Lib/ufo2ft/featureWriters/variableKernWriter.py

@@ -208,6 +208,15 @@ def getKerningPairs(
 
         return result
 
+    def _filterSpacingMarks(self, marks):
+        spacing = []
+        for mark in marks:
+            if all(
+                source.font[mark].width != 0 for source in self.context.font.sources
+            ):
+                spacing.append(mark)
+        return spacing
+
 
 def log_redefined_group(
     side: str, name: str, group: tuple[str, ...], font: Any, members: set[str]