Base class for all pop-up UI elements. Pop-ups are placed on top of all other UI, but are not widgets. Some pop-ups can be opened only once.
Hides the popup.
Makes the popup visible.
This function is called to create JSX popup elements. You may override it in your own subclass to create custom JSX behavior. The function is not called with any context, i.e.
this is not available.
This property can only be set via constructor. Once set, it cannot change anymore.