warn when building COLRv0 font if gid1 is not blank

Lib/ufo2ft/outlineCompiler.py

@@ -988,13 +988,31 @@ def setupTable_COLR(self):
                 for glyphs, box in self.ufo.lib.get(COLR_CLIP_BOXES_KEY, ())
                 for glyphName in glyphs
             }
-            self.otf["COLR"] = buildCOLR(
+            colr = buildCOLR(
                 layerInfo,
                 glyphMap=glyphMap,
                 clipBoxes=clipBoxes,
                 allowLayerReuse=self.colrLayerReuse,
             )
 
+            # Warn if there are any COLRv0 base glyphs and the gid1 isn't blank
+            # https://github.com/MicrosoftDocs/typography-issues/issues/346
+            if (colr.version == 0 or colr.table.BaseGlyphRecordCount > 0) and len(
+                self.glyphOrder
+            ) > 1:
+                g1 = self.allGlyphs[self.glyphOrder[1]]
+                if len(g1) > 0 or len(g1.components) > 0:
+                    logger.warning(
+                        "COLRv0 might not render correctly on Windows because "
+                        "the glyph at index 1 is not empty ('%s'). DirectWrite's "
+                        "COLRv0 implementation in Windows 10 used to require glyph ID "
+                        "1 to be blank, see: "
+                        "https://github.com/MicrosoftDocs/typography-issues/issues/346",
+                        g1.name,
+                    )
+
+            self.otf["COLR"] = colr
+
     def _computeCOLRClipBoxes(self):
         if (
             "COLR" not in self.otf

tests/outlineCompiler_test.py

@@ -1009,6 +1009,21 @@ def test_colr_cpal_interpolatable_ttf(self, FontClass):
             "c": [("c.color2", 1), ("c.color1", 0)],
         }
 
+    def test_colr_cpal_gid1_not_blank(self, FontClass, caplog):
+        # https://github.com/MicrosoftDocs/typography-issues/issues/346
+        testufo = FontClass(getpath("ColorTest.ufo"))
+        del testufo["space"]
+
+        with caplog.at_level(logging.WARNING, logger="ufo2ft.outlineCompiler"):
+            ttf = compileTTF(testufo)
+
+        assert ttf["COLR"].version == 0
+        assert ttf.getGlyphOrder()[1] == "a"
+        assert (
+            "COLRv0 might not render correctly on Windows because "
+            "the glyph at index 1 is not empty ('a')."
+        ) in caplog.text
+
     @pytest.mark.parametrize("compileFunc", [compileTTF, compileOTF])
     @pytest.mark.parametrize("manualClipBoxes", [True, False])
     @pytest.mark.parametrize(