diff --git a/OpenRa.Game/MainWindow.cs b/OpenRa.Game/MainWindow.cs
index 6270438ae6..af3d908477 100644
--- a/OpenRa.Game/MainWindow.cs
+++ b/OpenRa.Game/MainWindow.cs
@@ -41,12 +41,13 @@ namespace OpenRa.Game
 			Visible = true;
 
 			renderer = new Renderer(this, GetResolution(settings), false);
-			viewport = new Viewport(ClientSize);
-
-			SheetBuilder.Initialize(renderer.Device);
 
 			map = new Map(new IniFile(File.OpenRead("../../../" + settings.GetValue("map", "scm12ea.ini"))));
 
+			viewport = new Viewport(ClientSize, new float2(map.Size));
+
+			SheetBuilder.Initialize(renderer.Device);
+
 			TileMix = new Package("../../../" + map.Theater + ".mix");
 
 			renderer.SetPalette(new HardwarePalette(renderer.Device, map));
@@ -91,14 +92,10 @@ namespace OpenRa.Game
 			if (e.Button == 0)
 				return;
 
-			float2 scrollPos = new float2(viewport.ScrollPosition) + lastPos - new float2(e.Location);
-			float2 mapSize = 24 * new float2(map.Size) - viewport.Size + new float2(128, 0);
+			float2 p = new float2(e.Location);
 
-			scrollPos = scrollPos.Constrain(new Range<float2>(float2.Zero, mapSize));
-
-			lastPos = new float2(e.Location);
-
-			viewport.ScrollPosition = scrollPos.ToPointF();
+			viewport.Scroll(lastPos - p);
+			lastPos = p;
 		}
 
 		void Frame()
diff --git a/OpenRa.Game/Viewport.cs b/OpenRa.Game/Viewport.cs
index 99e4091f0d..38d1eadf75 100644
--- a/OpenRa.Game/Viewport.cs
+++ b/OpenRa.Game/Viewport.cs
@@ -2,18 +2,26 @@ using System;
 using System.Collections.Generic;
 using System.Text;
 using System.Drawing;
+using BluntDirectX.Direct3D;
 
 namespace OpenRa.Game
 {
 	class Viewport
 	{
 		readonly Size clientSize;
+		readonly float2 mapSize;
 		PointF scrollPosition;
 
 		public PointF ScrollPosition
 		{
 			get { return scrollPosition; }
-			set { scrollPosition = value; }
+		}
+
+		public void Scroll(float2 delta)
+		{
+			float2 scrollPos = new float2(ScrollPosition) + delta;
+			scrollPos = scrollPos.Constrain(new Range<float2>(float2.Zero, mapSize));
+			scrollPosition = scrollPos.ToPointF();
 		}
 
 		public Size ClientSize
@@ -21,9 +29,10 @@ namespace OpenRa.Game
 			get { return clientSize; }
 		} 
 
-		public Viewport(Size clientSize)
+		public Viewport(Size clientSize, float2 mapSize)
 		{
 			this.clientSize = clientSize;
+			this.mapSize = 24 * mapSize - Size + new float2(128, 0);
 		}
 
 		public float2 Size { get { return new float2(clientSize); } }