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>