I have a problem with masking the viewport or content panel. Right now in my application, I have a button that when I clicked it, it will call the setUrl method in the content panel (or TabItem). What I want to achieve is, when the TabItem loading the URL, the parent container will set to mask (so the user can't do anything with the application). The problem is I can't find any event to listen when the TabItem loading the URL. Is there anyone who has the same experience / problem? Please kindly advice me.