code: function addHandler( handler, targetState ) {
              ( targetState ? showHandlers : hideHandlers ).push( handler );
  
              // State already known to be true? In that case, initialize:
              if( targetState && axVisibility.isVisible() === targetState ) {
                 handler( targetState );
              }
           }
docs: eslint-disable-next-line valid-jsdoc 
  Add a show/hide-handler for a given area and visibility state. Execute the handler right away if
  the state is already known, and `true` (since all widgets start as invisible).
  @private