Fix the Fill computations for layout, by only special casing the Pos.Center scenarios and unrolling the rest

Designer/Program.cs

@@ -0,0 +1,45 @@
+using System;
+using Terminal.Gui;
+
+namespace Designer {
+	class Surface : Window {
+		public Surface () : base ("Designer")
+		{
+		}
+	}
+
+	class MainClass {
+		public static void Main (string [] args)
+		{
+			Application.Init ();
+
+			var menu = new MenuBar (new MenuBarItem [] {
+				new MenuBarItem ("_File", new MenuItem [] {
+					new MenuItem ("_Quit", "", () => { Application.RequestStop (); })
+				}),
+				new MenuBarItem ("_Edit", new MenuItem [] {
+					new MenuItem ("_Copy", "", null),
+					new MenuItem ("C_ut", "", null),
+					new MenuItem ("_Paste", "", null)
+				}),
+			});
+
+			var login = new Label ("Login: ") { X = 3, Y = 6 };
+			var password = new Label ("Password: ") {
+				X = Pos.Left (login),
+				Y = Pos.Bottom (login) + 1
+			};
+
+			var surface = new Surface () {
+				X = 0,
+				Y = 1,
+				Width = Dim.Percent (80),
+				Height = Dim.Fill ()
+			};
+
+			//Application.Top.Add (menu);
+			Application.Top.Add (login, password);
+			Application.Run ();
+		}
+	}
+}