Only crop GIF frames when saving with disposal method 2 if transparency is present

Tests/test_file_gif.py

@@ -762,6 +762,21 @@ def test_dispose2_previous_frame(tmp_path: Path) -> None:
         assert im.getpixel((0, 0)) == (0, 0, 0, 255)
 
 
+def test_dispose2_without_transparency(tmp_path: Path) -> None:
+    out = str(tmp_path / "temp.gif")
+
+    im = Image.new("P", (100, 100))
+
+    im2 = Image.new("P", (100, 100), (0, 0, 0))
+    im2.putpixel((50, 50), (255, 0, 0))
+
+    im.save(out, save_all=True, append_images=[im2], disposal=2)
+
+    with Image.open(out) as reloaded:
+        reloaded.seek(1)
+        assert reloaded.tile[0].extents == (0, 0, 100, 100)
+
+
 def test_transparency_in_second_frame(tmp_path: Path) -> None:
     out = str(tmp_path / "temp.gif")
     with Image.open("Tests/images/different_transparency.gif") as im:

src/PIL/GifImagePlugin.py

@@ -689,16 +689,21 @@ def _write_multiple_frames(
                         im_frames[-1].encoderinfo["duration"] += encoderinfo["duration"]
                     continue
                 if im_frames[-1].encoderinfo.get("disposal") == 2:
-                    if background_im is None:
-                        color = im.encoderinfo.get(
-                            "transparency", im.info.get("transparency", (0, 0, 0))
-                        )
-                        background = _get_background(im_frame, color)
-                        background_im = Image.new("P", im_frame.size, background)
-                        first_palette = im_frames[0].im.palette
-                        assert first_palette is not None
-                        background_im.putpalette(first_palette, first_palette.mode)
-                    bbox = _getbbox(background_im, im_frame)[1]
+                    # To appear correctly in viewers using a convention,
+                    # only consider transparency, and not background color
+                    color = im.encoderinfo.get(
+                        "transparency", im.info.get("transparency")
+                    )
+                    if color is not None:
+                        if background_im is None:
+                            background = _get_background(im_frame, color)
+                            background_im = Image.new("P", im_frame.size, background)
+                            first_palette = im_frames[0].im.palette
+                            assert first_palette is not None
+                            background_im.putpalette(first_palette, first_palette.mode)
+                        bbox = _getbbox(background_im, im_frame)[1]
+                    else:
+                        bbox = (0, 0) + im_frame.size
                 elif encoderinfo.get("optimize") and im_frame.mode != "1":
                     if "transparency" not in encoderinfo:
                         assert im_frame.palette is not None
@@ -764,7 +769,8 @@ def _write_multiple_frames(
             if not palette:
                 frame_data.encoderinfo["include_color_table"] = True
 
-            im_frame = im_frame.crop(frame_data.bbox)
+            if frame_data.bbox != (0, 0) + im_frame.size:
+                im_frame = im_frame.crop(frame_data.bbox)
             offset = frame_data.bbox[:2]
         _write_frame_data(fp, im_frame, offset, frame_data.encoderinfo)
     return True