easyui menubutton combobox 被遮盖问题

如图一所示，menubutton 中的 combobox 被遮盖 z-Index 不够。这是作者给出的解决方案

 <a href="#" class="easyui-menubutton" data-options="menu:'#mm1'">Search</a>

 <div id="mm1" class="menu-content" data-options="hideOnUnhover:false">

     <select class="easyui-combobox" data-options="

             onShowPanel:function(){

                 $(this).combobox('panel').panel('panel').css('z-index',$.fn.menu.defaults.zIndex++);

             }

             ">

         <option>Item1</option>

         <option>Item2</option>

         <option>Item3</option>

     </select>

 </div>

 <script>

 $(function(){

     $(document).unbind('.combo').unbind('.menu')

         .bind('mousedown.combo', handler)

         .bind('mousedown.menu', handler);

     function handler(e){

         var d = $(e.target).closest('span.combo,div.combo-p,div.menu');

         if (d.hasClass('combo-p')){

             d.find('.combo-f').each(function(){

                 var p = $(this).combo('panel');

                 if (p.is(':visible')){

                     p.panel('close');

                 }

             });

             return;

         } else if (d.hasClass('combo')){

             return;

         } else if (d.hasClass('menu')){

             return;

         } else {

             $('body>div.combo-p>div.combo-panel:visible').panel('close');

             $('body>div.menu-top:visible').menu('hide');

         }

     }

 });

 </script>