Fix size calculation of Image.thumbnail()

Tests/test_image_thumbnail.py

@@ -50,6 +50,14 @@ def test_aspect():
     im.thumbnail((33, 33))
     assert im.size == (21, 33)  # ratio is 0.6363636364
 
+    im = Image.new("L", (145, 100))  # ratio is 1.45
+    im.thumbnail((50, 50))
+    assert im.size ==(50, 34)  # ratio is 1.47058823529
+
+    im = Image.new("L", (100, 145))  # ratio is 0.689655172414
+    im.thumbnail((50, 50))
+    assert im.size == (34, 50)  # ratio is 0.68
+
 
 def test_float():
     im = Image.new("L", (128, 128))

src/PIL/Image.py

@@ -2237,19 +2237,18 @@ class Image:
         """
 
         # preserve aspect ratio
-        x, y = self.size
-        if x > size[0]:
-            y = max(round(y * size[0] / x), 1)
-            x = round(size[0])
-        if y > size[1]:
-            x = max(round(x * size[1] / y), 1)
-            y = round(size[1])
-        size = x, y
-        box = None
+        x, y = size
+        aspect = self.width / self.height
+        if x / y >= aspect:
+            x = max(y * aspect, 1)
+        else:
+            y = max(x / aspect, 1)
+        size = (round(x), round(y))
 
         if size == self.size:
             return
 
+        box = None
         if reducing_gap is not None:
             res = self.draft(None, (size[0] * reducing_gap, size[1] * reducing_gap))
             if res is not None: