Treat previous contents of first frame as transparent

Tests/test_file_gif.py

@@ -341,6 +341,14 @@ def test_dispose_previous():
             pass
 
 
+def test_dispose_previous_first_frame():
+    with Image.open("Tests/images/dispose_prev_first_frame.gif") as im:
+        im.seek(1)
+        assert_image_equal_tofile(
+            im, "Tests/images/dispose_prev_first_frame_seeked.gif"
+        )
+
+
 def test_previous_frame_loaded():
     with Image.open("Tests/images/dispose_none.gif") as img:
         img.load()

src/PIL/GifImagePlugin.py

@@ -277,6 +277,14 @@ class GifImageFile(ImageFile.ImageFile):
                 if self.im:
                     # only dispose the extent in this frame
                     self.dispose = self._crop(self.im, self.dispose_extent)
+                elif frame_transparency is not None:
+                    x0, y0, x1, y1 = self.dispose_extent
+                    dispose_size = (x1 - x0, y1 - y0)
+
+                    Image._decompression_bomb_check(dispose_size)
+                    self.dispose = Image.core.fill(
+                        "P", dispose_size, frame_transparency
+                    )
         except AttributeError:
             pass