Expand C image to match GIF frame image size

Tests/test_file_gif.py

@@ -1378,16 +1378,37 @@ def test_lzw_bits() -> None:
         im.load()
 
 
-def test_extents() -> None:
-    with Image.open("Tests/images/test_extents.gif") as im:
-        assert im.size == (100, 100)
+@pytest.mark.parametrize(
+    "test_file, loading_strategy",
+    (
+        ("test_extents.gif", GifImagePlugin.LoadingStrategy.RGB_AFTER_FIRST),
+        (
+            "test_extents.gif",
+            GifImagePlugin.LoadingStrategy.RGB_AFTER_DIFFERENT_PALETTE_ONLY,
+        ),
+        (
+            "test_extents_transparency.gif",
+            GifImagePlugin.LoadingStrategy.RGB_AFTER_FIRST,
+        ),
+    ),
+)
+def test_extents(test_file, loading_strategy) -> None:
+    GifImagePlugin.LOADING_STRATEGY = loading_strategy
+    try:
+        with Image.open("Tests/images/" + test_file) as im:
+            assert im.size == (100, 100)
 
-        # Check that n_frames does not change the size
-        assert im.n_frames == 2
-        assert im.size == (100, 100)
+            # Check that n_frames does not change the size
+            assert im.n_frames == 2
+            assert im.size == (100, 100)
 
-        im.seek(1)
-        assert im.size == (150, 150)
+            im.seek(1)
+            assert im.size == (150, 150)
+
+            im.load()
+            assert im.im.size == (150, 150)
+    finally:
+        GifImagePlugin.LOADING_STRATEGY = GifImagePlugin.LoadingStrategy.RGB_AFTER_FIRST
 
 
 def test_missing_background() -> None:

src/PIL/GifImagePlugin.py

@@ -438,6 +438,13 @@ def load_prepare(self) -> None:
                 self.im.putpalette("RGB", *self._frame_palette.getdata())
             else:
                 self._im = None
+        if not self._prev_im and self._im is not None and self.size != self.im.size:
+            expanded_im = Image.core.fill(self.im.mode, self.size)
+            if self._frame_palette:
+                expanded_im.putpalette("RGB", *self._frame_palette.getdata())
+            expanded_im.paste(self.im, (0, 0) + self.im.size)
+
+            self.im = expanded_im
         self._mode = temp_mode
         self._frame_palette = None
 
@@ -455,6 +462,17 @@ def load_end(self) -> None:
             return
         if not self._prev_im:
             return
+        if self.size != self._prev_im.size:
+            if self._frame_transparency is not None:
+                expanded_im = Image.core.fill("RGBA", self.size)
+            else:
+                expanded_im = Image.core.fill("P", self.size)
+                expanded_im.putpalette("RGB", "RGB", self.im.getpalette())
+                expanded_im = expanded_im.convert("RGB")
+            expanded_im.paste(self._prev_im, (0, 0) + self._prev_im.size)
+
+            self._prev_im = expanded_im
+            assert self._prev_im is not None
         if self._frame_transparency is not None:
             self.im.putpalettealpha(self._frame_transparency, 0)
             frame_im = self.im.convert("RGBA")