Index: src/gui.d
==================================================================
--- src/gui.d
+++ src/gui.d
@@ -24,10 +24,11 @@
 			t.interval = 100;
 			t.tick ~= (Timer sender, EventArgs ea){
 				fn(solver.single_step());
 			};
 			t.start();
+			this.closing ~= (Form f,CancelEventArgs c){t.stop();};
 		}
 		Application.run(this);
 	}
 
 	override void on_game_changed(char c, const(Game) g, bool finished)