AnimGIF not transparent

[doublex]

The frames of the down below animated GIF are not transparent (first frame works).

Testcase:

import base64, io
import PIL.Image

animgif = '''
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with PIL.Image.open(io.BytesIO(base64.b64decode(animgif)), 'r') as img:
    for index, frame in enumerate(PIL.ImageSequence.Iterator(img)):
        img.save('/tmp/not-transparent-'+str(index)+'.png', 'PNG')

[radarhere]

Experimenting, I find that prioritising the global transparency over frame transparency when replacing with background color fixes this.

diff --git a/src/PIL/GifImagePlugin.py b/src/PIL/GifImagePlugin.py
index db6944267..128afc428 100644
--- a/src/PIL/GifImagePlugin.py
+++ b/src/PIL/GifImagePlugin.py
@@ -271,11 +271,9 @@ class GifImageFile(ImageFile.ImageFile):
                 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

Is the image data that you've provided able to be included as part of our test suite, and distributed under our Pillow license?

[doublex]

@radarhere
Thanks a lot for all your efforts.
Yes, the GIF image is license free (created in house).

[radarhere]

I've created PR #5756 to resolve this.