Fix adding a new color when image palette has missing background or transparency index (#9799)

Signed-off-by: Andrew Chen <48723787+chuenchen309@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Andrew Murray <radarhere@users.noreply.github.com>

Author: 3 people

Tests/test_imagepalette.py

@@ -87,13 +87,17 @@ def test_getcolor_not_special(index: int, palette: ImagePalette.ImagePalette) ->
 
     # Do not use transparency index as a new color
     im.info["transparency"] = index
-    index1 = palette.getcolor((0, 0, 0), im)
+    index1 = palette.getcolor((0, 0, 1), im)
     assert index1 != index
+    roundtripped_palette = ImagePalette.ImagePalette(palette=palette.palette)
+    assert roundtripped_palette.colors[(0, 0, 1)] == index1
 
     # Do not use background index as a new color
     im.info["background"] = index1
-    index2 = palette.getcolor((0, 0, 1), im)
+    index2 = palette.getcolor((0, 0, 2), im)
     assert index2 not in (index, index1)
+    roundtripped_palette = ImagePalette.ImagePalette(palette=palette.palette)
+    assert roundtripped_palette.colors[(0, 0, 2)] == index2
 
 
 def test_file(tmp_path: Path) -> None:

src/PIL/ImagePalette.py

@@ -125,7 +125,11 @@ def _new_color_index(
                 image.info.get("background"),
                 image.info.get("transparency"),
             )
+            assert isinstance(self._palette, bytearray)
             while index in special_colors:
+                # Background or transparency index points past the end of the palette.
+                # Set it to black, so that the new color can be written afterwards.
+                self._palette += bytearray(len(self.mode))
                 index += 1
         if index >= 256:
             if image: