Fixes #825. ReadOnly CheckBox.

Terminal.Gui/Views/Checkbox.cs

@@ -147,7 +147,7 @@ namespace Terminal.Gui {
 		///<inheritdoc/>
 		public override bool MouseEvent (MouseEvent me)
 		{
-			if (!me.Flags.HasFlag (MouseFlags.Button1Clicked))
+			if (!me.Flags.HasFlag (MouseFlags.Button1Clicked) || !CanFocus)
 				return false;
 
 			SuperView.SetFocus (this);