scroll maplist to the top when changing filter

OpenRA.Game/Widgets/ScrollPanelWidget.cs

@@ -125,6 +125,11 @@ namespace OpenRA.Widgets
 			ListOffset = Math.Min(0,Bounds.Height - ContentHeight);
 		}
 
+		public void ScrollToTop()
+		{
+			ListOffset = 0;
+		}
+
 		public override void Tick ()
 		{
 			if (UpPressed) Scroll(1);

OpenRA.Mods.RA/Widgets/Logic/MapChooserLogic.cs

@@ -80,6 +80,7 @@ namespace OpenRA.Mods.RA.Widgets.Logic
 		void EnumerateMaps()
 		{
 			scrollpanel.RemoveChildren();
+			(scrollpanel as ScrollPanelWidget).ScrollToTop();
 
 			var maps = Game.modData.AvailableMaps
 				.Where(kv => kv.Value.Selectable)