diff --git a/OpenRa.Game/Sidebar.cs b/OpenRa.Game/Sidebar.cs
index 6802ca2493..d316ed6669 100644
--- a/OpenRa.Game/Sidebar.cs
+++ b/OpenRa.Game/Sidebar.cs
@@ -32,6 +32,7 @@ namespace OpenRa.Game
 		public Sidebar( Race race, Renderer renderer, Game game )
 		{
 			this.techTree = game.LocalPlayer.TechTree;
+			this.techTree.BuildableItemsChanged += (sender, e) => { PopulateItemList(); };
 			this.game = game;
 			region = GRegion.Create(game.viewport, DockStyle.Right, 128, Paint, MouseHandler);
 			game.viewport.AddRegion( region );
@@ -98,7 +99,7 @@ namespace OpenRa.Game
 
 		void Paint()
 		{
-			PopulateItemList();	// todo: do this less often, just when things actually change!
+//			PopulateItemList();	// todo: do this less often, just when things actually change!
 
 			foreach (SidebarItem i in items)
 				i.Paint(spriteRenderer, region.Location);
diff --git a/OpenRa.Game/SidebarItem.cs b/OpenRa.Game/SidebarItem.cs
index 9dd4e530bc..44a0594c12 100644
--- a/OpenRa.Game/SidebarItem.cs
+++ b/OpenRa.Game/SidebarItem.cs
@@ -9,6 +9,8 @@ namespace OpenRa.Game
 		public readonly float2 location;
 		readonly Sprite sprite;
 
+		public bool isBuilding = false;
+
 		public SidebarItem(Sprite s, Item item, int y)
 		{
 			this.techTreeItem = item;
diff --git a/OpenRa.Game/TechTree/TechTree.cs b/OpenRa.Game/TechTree/TechTree.cs
index df51478937..f38c4b2935 100644
--- a/OpenRa.Game/TechTree/TechTree.cs
+++ b/OpenRa.Game/TechTree/TechTree.cs
@@ -93,8 +93,16 @@ namespace OpenRa.TechTree
 
 		void CheckAll()
 		{
+			bool changed = false;
 			foreach (Item unit in objects.Values)
+			{
+				bool old = unit.CanBuild;
 				unit.CheckPrerequisites(built, currentRace);
+				if (old != unit.CanBuild)
+					changed = true;
+			}
+
+			if (changed) BuildableItemsChanged(this, EventArgs.Empty);
 		}
 
 		public IEnumerable<Item> BuildableItems
@@ -106,5 +114,7 @@ namespace OpenRa.TechTree
 						yield return b;
 			}
 		}
+
+		public event EventHandler BuildableItemsChanged = (sender, e) => { };
 	}
 }