Use current frame for transparency detection

Tests/test_file_webp.py

@@ -208,7 +208,7 @@ class TestFileWebp:
 
     def test_background_from_gif(self, tmp_path: Path) -> None:
         out_webp = tmp_path / "temp.webp"
-        
+
         # Save L mode GIF with background
         with Image.open("Tests/images/no_palette_with_background.gif") as im:
             im.save(out_webp, save_all=True)

src/PIL/WebPImagePlugin.py

@@ -243,7 +243,9 @@ def _save_all(im: Image.Image, fp: IO[bytes], filename: str | bytes) -> None:
                 # Make sure image mode is supported
                 frame = ims
                 if frame.mode not in ("RGBX", "RGBA", "RGB"):
-                    frame = frame.convert("RGBA" if im.has_transparency_data else "RGB")
+                    frame = frame.convert(
+                        "RGBA" if frame.has_transparency_data else "RGB"
+                    )
 
                 # Append the frame to the animation encoder
                 enc.add(