When adding user_data in callbacks, it is difficult to make sure that the user_data is deleted when the widget is deleted, especially when deleting a group auto deletes all children.

Solution:

Add FL_WHEN_DELETING to Fl_Widget::when(), so before deleting a widget, the user can receive a callback and get a chance to clean up. The reason for the callback would be indicated by setting Fl::event() to FL_DELETING, which would also be sent to Fl_Widget::handle(int).