From a47072bcbe117324a2373cc96d733f07bee267d7 Mon Sep 17 00:00:00 2001
From: Andrew Murray <radarhere@users.noreply.github.com>
Date: Thu, 15 Apr 2021 19:01:12 +1000
Subject: [PATCH] Treat previous contents of first frame as transparent

---
 Tests/images/dispose_prev_first_frame.gif        | Bin 0 -> 530 bytes
 Tests/images/dispose_prev_first_frame_seeked.gif | Bin 0 -> 1028 bytes
 Tests/test_file_gif.py                           |   8 ++++++++
 src/PIL/GifImagePlugin.py                        |   8 ++++++++
 4 files changed, 16 insertions(+)
 create mode 100644 Tests/images/dispose_prev_first_frame.gif
 create mode 100644 Tests/images/dispose_prev_first_frame_seeked.gif

diff --git a/Tests/images/dispose_prev_first_frame.gif b/Tests/images/dispose_prev_first_frame.gif
new file mode 100644
index 0000000000000000000000000000000000000000..4c19dd1ed43d2347a7914062e63d8dcfd1f55d4a
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diff --git a/Tests/images/dispose_prev_first_frame_seeked.gif b/Tests/images/dispose_prev_first_frame_seeked.gif
new file mode 100644
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diff --git a/Tests/test_file_gif.py b/Tests/test_file_gif.py
index 6d92bde76..df029dceb 100644
--- a/Tests/test_file_gif.py
+++ b/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()
diff --git a/src/PIL/GifImagePlugin.py b/src/PIL/GifImagePlugin.py
index 3c4cb6187..5c93de2c9 100644
--- a/src/PIL/GifImagePlugin.py
+++ b/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