popover 报错，显示不正常

看css 遮罩层 Z-index 是998
popover z-index 999 为啥还显示在下面

<!--主界面部分-->  
<div class="mui-inner-wrap">  
    <header class="mui-bar mui-bar-nav">  
        <a href="#offCanvasSide" class="mui-icon mui-action-menu mui-icon-bars mui-pull-left"></a>  
        <a class="mui-action-menu mui-icon mui-icon-search mui-pull-right" href="#popover"></a>  
        <h1 class="mui-title">经营报表</h1>  
    </header>  
    <div id="offCanvasContentScroll" class="mui-content mui-scroll-wrapper">  
        <div class="mui-content-padded">  

        </div>  

    </div>  
    <div id="popover" class="mui-popover">  
        <ul class="mui-table-view">  
            <li class="mui-table-view-cell"><a href="#">Item1</a></li>  
            <li class="mui-table-view-cell"><a href="#">Item2</a></li>  
            <li class="mui-table-view-cell"><a href="#">Item3</a></li>  
            <li class="mui-table-view-cell"><a href="#">Item4</a></li>  
            <li class="mui-table-view-cell"><a href="#">Item5</a></li>  
        </ul>  
    </div>  
</div>