Add commentary for fix in PR#442

synapse/rest/client/v2_alpha/sync.py

@@ -356,6 +356,12 @@ class SyncRestServlet(RestServlet):
 
             prev_content = timeline_event.unsigned.get('prev_content')
             prev_sender = timeline_event.unsigned.get('prev_sender')
+            # Empircally it seems possible for the event to have a
+            # "replaces_state" key but not a prev_content or prev_sender
+            # markjh conjectures that it could be due to the server not
+            # having a copy of that event.
+            # If this is the case the we ignore the previous event. This will
+            # cause the displayname calculations on the client to be incorrect
             if prev_event_id is None or not prev_content or not prev_sender:
                 del result[event_key]
             else: