1 files changed, 17 insertions, 9 deletions
diff --git a/synapse/rest/media/v1/thumbnail_resource.py b/synapse/rest/media/v1/thumbnail_resource.py
index a029d426f0..12bd745cb2 100644
--- a/synapse/rest/media/v1/thumbnail_resource.py
+++ b/synapse/rest/media/v1/thumbnail_resource.py
@@ -15,7 +15,7 @@
 
 
 import logging
-from typing import TYPE_CHECKING, Any, Dict, List, Optional
+from typing import TYPE_CHECKING, Any, Dict, List, Optional, Tuple
 
 from twisted.web.server import Request
 
@@ -414,9 +414,9 @@ class ThumbnailResource(DirectServeJsonResource):
 
         if desired_method == "crop":
             # Thumbnails that match equal or larger sizes of desired width/height.
-            crop_info_list = []
+            crop_info_list: List[Tuple[int, int, int, bool, int, Dict[str, Any]]] = []
             # Other thumbnails.
-            crop_info_list2 = []
+            crop_info_list2: List[Tuple[int, int, int, bool, int, Dict[str, Any]]] = []
             for info in thumbnail_infos:
                 # Skip thumbnails generated with different methods.
                 if info["thumbnail_method"] != "crop":
@@ -451,15 +451,19 @@ class ThumbnailResource(DirectServeJsonResource):
                             info,
                         )
                     )
+            # Pick the most appropriate thumbnail. Some values of `desired_width` and
+            # `desired_height` may result in a tie, in which case we avoid comparing on
+            # the thumbnail info dictionary and pick the thumbnail that appears earlier
+            # in the list of candidates.
             if crop_info_list:
-                thumbnail_info = min(crop_info_list)[-1]
+                thumbnail_info = min(crop_info_list, key=lambda t: t[:-1])[-1]
             elif crop_info_list2:
-                thumbnail_info = min(crop_info_list2)[-1]
+                thumbnail_info = min(crop_info_list2, key=lambda t: t[:-1])[-1]
         elif desired_method == "scale":
             # Thumbnails that match equal or larger sizes of desired width/height.
-            info_list = []
+            info_list: List[Tuple[int, bool, int, Dict[str, Any]]] = []
             # Other thumbnails.
-            info_list2 = []
+            info_list2: List[Tuple[int, bool, int, Dict[str, Any]]] = []
 
             for info in thumbnail_infos:
                 # Skip thumbnails generated with different methods.
@@ -477,10 +481,14 @@ class ThumbnailResource(DirectServeJsonResource):
                     info_list2.append(
                         (size_quality, type_quality, length_quality, info)
                     )
+            # Pick the most appropriate thumbnail. Some values of `desired_width` and
+            # `desired_height` may result in a tie, in which case we avoid comparing on
+            # the thumbnail info dictionary and pick the thumbnail that appears earlier
+            # in the list of candidates.
             if info_list:
-                thumbnail_info = min(info_list)[-1]
+                thumbnail_info = min(info_list, key=lambda t: t[:-1])[-1]
             elif info_list2:
-                thumbnail_info = min(info_list2)[-1]
+                thumbnail_info = min(info_list2, key=lambda t: t[:-1])[-1]
 
         if thumbnail_info:
             return FileInfo(