colorLib.unbuilder: ensure unbuildColrV1 flattens nested PaintColrLayers

Lib/fontTools/colorLib/unbuilder.py

@@ -13,12 +13,12 @@ def unbuildColrV1(layerList, baseGlyphList):
     }
 
 
-def _flatten(lst):
+def _flatten_layers(lst):
-    for el in lst:
+    for paint in lst:
-        if isinstance(el, list):
+        if paint["Format"] == ot.PaintFormat.PaintColrLayers:
-            yield from _flatten(el)
+            yield from _flatten_layers(paint["Layers"])
         else:
-            yield el
+            yield paint
 
 
 class LayerListUnbuilder:
@@ -41,7 +41,7 @@ class LayerListUnbuilder:
         assert source["Format"] == ot.PaintFormat.PaintColrLayers
 
         layers = list(
-            _flatten(
+            _flatten_layers(
                 [
                     self.unbuildPaint(childPaint)
                     for childPaint in self.layers[

Tests/colorLib/unbuilder_test.py

@@ -221,7 +221,26 @@ TEST_COLOR_GLYPHS = {
                 "Glyph": "glyph00012",
             },
         ],
+    },
+    # When PaintColrLayers contains more than 255 layers, we build a tree
+    # of nested PaintColrLayers of max 255 items (NumLayers field is a uint8).
+    # Below we test that unbuildColrV1 restores a flat list of layers without
+    # nested PaintColrLayers.
+    "glyph00017": {
+        "Format": int(ot.PaintFormat.PaintColrLayers),
+        "Layers": [
+            {
+                "Format": int(ot.PaintFormat.PaintGlyph),
+                "Paint": {
+                    "Format": int(ot.PaintFormat.PaintSolid),
+                    "PaletteIndex": i,
+                    "Alpha": 1.0,
+                },
+                "Glyph": "glyph{str(18 + i).zfill(5)}",
             }
+            for i in range(256)
+        ],
+    },
 }
 
 
@@ -230,6 +249,7 @@ def test_unbuildColrV1():
     colorGlyphs = unbuildColrV1(layers, baseGlyphs)
     assert colorGlyphs == TEST_COLOR_GLYPHS
 
+
 def test_unbuildColrV1_noLayers():
     _, baseGlyphsV1 = buildColrV1(TEST_COLOR_GLYPHS)
     # Just looking to see we don't crash