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>