src/lay/modules/form.js
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);
            }
          }
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);
            }
          }
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