Add option to *not* group mark classes within same mark lookup

Lib/ufo2ft/featureWriters/markFeatureWriter.py

@@ -266,9 +266,15 @@ class MarkFeatureWriter(BaseFeatureWriter):
 
     If the `quantization` argument is given in the filter options, the resulting
     anchors are rounded to the nearest multiple of the quantization value.
+
+    Mark-to-base or mark-to-ligature attachments that reference non-overlapping
+    mark classes get grouped in the same lookup. If a mark glyph is in more than
+    one mark class, then additional lookups are generated for those as required.
+    If you prefer to have as many mark-to-base and mark-to-liga lookups as there
+    are mark classes, you can disable this with `groupMarkClasses=False`.
     """
 
-    options = dict(quantization=1)
+    options = dict(quantization=1, groupMarkClasses=True)
 
     tableTag = "GPOS"
     features = frozenset(["mark", "mkmk", "abvm", "blwm"])
@@ -546,12 +552,24 @@ def _groupAttachments(self, attachments):
         #   through different anchor names, we may have to split the attachment
         #   into two attachments, using null anchors instead of one or the other
         #   mark class in each split attachment.
-        markGlyphToMarkClasses = defaultdict(set)
-        for attachment in attachments:
-            for markGlyph, markClasses in attachment.getMarkGlyphToMarkClasses():
-                markGlyphToMarkClasses[markGlyph].update(markClasses)
-        groupedMarkClasses = self._groupMarkClasses(markGlyphToMarkClasses)
-        self._logIfAmbiguous(attachments, groupedMarkClasses)
+        if self.options.groupMarkClasses:
+            markGlyphToMarkClasses = defaultdict(set)
+            for attachment in attachments:
+                for markGlyph, markClasses in attachment.getMarkGlyphToMarkClasses():
+                    markGlyphToMarkClasses[markGlyph].update(markClasses)
+            groupedMarkClasses = self._groupMarkClasses(markGlyphToMarkClasses)
+            self._logIfAmbiguous(attachments, groupedMarkClasses)
+        else:
+            # this will generate one lookup per mark class, and sort them
+            # lexicographically by the anchor name, so the lookup for e.g.
+            # '_top.alt01' will occur *after* the one for `_top` (the last wins) thus
+            # allowing some degree of control on potentially ambiguous attachments
+            # https://github.com/googlefonts/ufo2ft/issues/762
+            # https://github.com/googlefonts/ufo2ft/issues/591
+            groupedMarkClasses = [
+                [markClass.name]
+                for _, markClass in sorted(self.context.markClasses.items())
+            ]
         lookups = []
         for markClasses in groupedMarkClasses:
             lookup = []