Move this to its own file and inherit from EventArgs

Terminal.Gui/Application.cs

@@ -1843,8 +1843,4 @@ public static partial class Application
     }
 
     #endregion Keyboard handling
-}
-
-/// <summary>Event arguments for the <see cref="Application.Iteration"/> event.</summary>
-public class IterationEventArgs
-{ }
+}

Terminal.Gui/IterationEventArgs.cs

@@ -0,0 +1,5 @@
+namespace Terminal.Gui;
+
+/// <summary>Event arguments for the <see cref="Application.Iteration"/> event.</summary>
+public class IterationEventArgs : EventArgs
+{ }