Avoid virtual dispatch by returning the most-derived concrete type available

Terminal.Gui/Application.cs

@@ -1126,7 +1126,7 @@ public static partial class Application
     }
 
     #nullable enable
-    private static View? FindDeepestTop (Toplevel start, int x, int y)
+    private static Toplevel? FindDeepestTop (Toplevel start, int x, int y)
     {
         if (!start.Frame.Contains (x, y))
         {