tokens: In the end I took another approach ; instead of trying to stop the user from dragging ANY of the child control outside of the boundary of the container I have only stopped them from dragging the pointer outside the boundary , e.g . in the mousemove eventhandler :
ner_tags: O O O O O O O O O O O O O O O O O O O O O B-Class O O O O O O O O O O O O O O O B-User_Interface_Element O O O O O O O O B-Variable B-Class O