Test largest cursor

Tests/test_file_cur.py

@@ -1,8 +1,13 @@
 from __future__ import annotations
 
+from io import BytesIO
+
 import pytest
 
 from PIL import CurImagePlugin, Image
+from PIL._binary import o8
+from PIL._binary import o16le as o16
+from PIL._binary import o32le as o32
 
 TEST_FILE = "Tests/images/deerstalker.cur"
 
@@ -17,6 +22,24 @@ def test_sanity() -> None:
         assert im.getpixel((16, 16)) == (84, 87, 86, 255)
 
 
+def test_largest_cursor() -> None:
+    magic = b"\x00\x00\x02\x00"
+    sizes = ((1, 1), (8, 8), (4, 4))
+    data = magic + o16(len(sizes))
+    for w, h in sizes:
+        image_offset = 6 + len(sizes) * 16 if (w, h) == max(sizes) else 0
+        data += o8(w) + o8(h) + o8(0) * 10 + o32(image_offset)
+    data += (
+        o32(12)  # header size
+        + o16(8)  # width
+        + o16(16)  # height
+        + o16(0)  # planes
+        + o16(1)  # bits
+    )
+    with Image.open(BytesIO(data)) as im:
+        assert im.size == (8, 8)
+
+
 def test_invalid_file() -> None:
     invalid_file = "Tests/images/flower.jpg"