Prefer global transparency in GIF when replacing with background color

Tests/test_file_gif.py

@@ -337,6 +337,13 @@ def test_dispose_background():
             pass
 
 
+def test_dispose_background_transparency():
+    with Image.open("Tests/images/dispose_bgnd_transparency.gif") as img:
+        img.seek(2)
+        px = img.convert("RGBA").load()
+        assert px[35, 30][3] == 0
+
+
 def test_transparent_dispose():
     expected_colors = [(2, 1, 2), (0, 1, 0), (2, 1, 2)]
     with Image.open("Tests/images/transparent_dispose.gif") as img:

src/PIL/GifImagePlugin.py

@@ -271,11 +271,9 @@ def _seek(self, frame):
                 Image._decompression_bomb_check(dispose_size)
 
                 # by convention, attempt to use transparency first
-                color = (
-                    frame_transparency
-                    if frame_transparency is not None
-                    else self.info.get("background", 0)
-                )
+                color = self.info.get("transparency", frame_transparency)
+                if color is None:
+                    color = self.info.get("background", 0)
                 self.dispose = Image.core.fill("P", dispose_size, color)
             else:
                 # replace with previous contents