A Game-tick can execute viewport.Tick itself,

removed this dependency from World.

OpenRA.Game/Game.cs

@@ -188,6 +188,8 @@ namespace OpenRA
 						else
 							if (orderManager.NetFrameNumber == 0)
 								orderManager.LastTickTime = Environment.TickCount;
+					
+						viewport.Tick();
 					}
 				}
 		}

OpenRA.Game/World.cs

@@ -183,7 +183,7 @@ namespace OpenRA
 
 			while (frameEndActions.Count != 0)
 				frameEndActions.Dequeue()(this);
-			Game.viewport.Tick();
+			
 		}
 
 		public IEnumerable<Actor> Actors { get { return actors; } }