code: function(){  
              var thisDd = dl.children('dd.'+ THIS);
              
              if(!thisDd[0]) return;
              
              var posTop = thisDd.position().top
              ,dlHeight = dl.height()
              ,ddHeight = thisDd.height();
              
              //若选中元素在滚动条不可见底部
              if(posTop > dlHeight){
                dl.scrollTop(posTop + dl.scrollTop() - dlHeight + ddHeight - 5);
              }
              
              //若选择玄素在滚动条不可见顶部
              if(posTop < 0){
                dl.scrollTop(posTop + dl.scrollTop() - 5);
              }
            }
docs: 展开下拉