Implement "Link Metrics..." custom parameters

Lib/glyphsLib/builder/glyph.py

@@ -114,7 +114,53 @@ def to_ufo_glyph(self, ufo_glyph, layer, glyph):  # noqa: C901
         ufo_glyph.lib[GLYPHLIB_PREFIX + "subCategory"] = subCategory
 
     # load width before background, which is loaded with lib data
-    width = layer.width
+
+    # The width may be taken from another master via the customParameters
+    # 'Link Metrics With Master' or 'Link Metrics With First Master'.
+    master = self.font.masters[layer.associatedMasterId or layer.layerId]
+    source_master_id = master.customParameters[
+        "Link Metrics With Master"
+    ]
+    if source_master_id is None:
+        user_first_master = master.customParameters[
+            "Link Metrics With First Master"
+        ]
+        if user_first_master == 1:
+            width = self.font.glyphs[glyph.name].layers[0].width
+            if layer.width != width:
+                logger.info(
+                    f"{layer.parent.name}: Applying width from first master: "
+                    f"{layer.width} -> {width}"
+                )
+        else:
+            width = layer.width
+    else:
+        source_layer = self.font.glyphs[glyph.name].layers[source_master_id]
+        if source_layer is None:
+            # Try by name instead of master ID
+            found = False
+            for master in self.font.masters:
+                if master.name == source_master_id:
+                    found = True
+                    source_layer = self.font.glyphs[glyph.name].layers[master.id]
+                    break
+        else:
+            found = True
+        if found:
+            width = source_layer.width
+        else:
+            logger.debug(
+                f"{layer.parent.name}: Trying to apply width from master "
+                f"'{source_master_id}': master not found."
+            )
+            # Use the current layer width
+            width = layer.width
+        if layer.width != width:
+            logger.info(
+                f"{layer.parent.name}: Applying width from master "
+                f"'{source_master_id}': {layer.width} -> {width}"
+            )
+
     if width is None:
         pass
     elif category == "Mark" and subCategory == "Nonspacing" and width > 0:

Lib/glyphsLib/builder/kerning.py

@@ -15,12 +15,50 @@
 
 import re
 
+from copy import deepcopy
+
 UFO_KERN_GROUP_PATTERN = re.compile("^public\\.kern([12])\\.(.*)$")
 
 
 def to_ufo_kerning(self):
-    for master_id, kerning in self.font.kerning.items():
-        _to_ufo_kerning(self, self._sources[master_id].font, kerning)
+    # Check if the kerning should be copied from another master
+    kerning_source_masters = {}
+
+    for master in self.font.masters:
+        master_id = master.id
+        master = self.font.masters[master_id]
+        source_master_id = master.customParameters[
+            "Link Metrics With Master"
+        ]
+        if source_master_id is None:
+            source_master_id = master.customParameters[
+                "Link Metrics With First Master"
+            ]
+            if source_master_id == 1:
+                kerning_source_masters[master_id] = self.font.masters[0].id
+        else:
+            source_master = self.font.masters[source_master_id]
+            if source_master is None:
+                # Try by name instead of master ID
+                found = False
+                for source_master in self.font.masters:
+                    if source_master.name == source_master_id:
+                        found = True
+                        source_master_id = source_master.id
+                        break
+            else:
+                found = True
+            if found:
+                kerning_source_masters[master_id] = source_master_id
+            else:
+                print(f"Source master for kerning not found: '{source_master_id}'")
+
+    for master in self.font.masters:
+        master_id = master.id
+        source_master_id = kerning_source_masters.get(master_id, master_id)
+        if source_master_id in self.font.kerning:
+            kerning = deepcopy(self.font.kerning[source_master_id])
+            _to_ufo_kerning(self, self._sources[master_id].font, kerning)
 
 
 def _to_ufo_kerning(self, ufo, kerning_data):