code: function updateState( targetState ) {
              const state = axVisibility.isVisible();
              if( state === lastState ) {
                 return;
              }
              lastState = state;
              heartbeat.onAfterNext( () => {
                 const handlers = targetState ? showHandlers : hideHandlers;
                 handlers.forEach( f => f( targetState ) );
              } );
           }
docs: eslint-disable-next-line valid-jsdoc 
  Run all handlers registered for the given area and target state after the next heartbeat.
  Also remove any handlers that have been cleared since the last run.
  @private