隐藏元素被限高了，怎么办？_html/css_WEB-ITnose

.top_b_wrap{ width:100%; height:auto; overflow:hidden;  background:#c9c9c9;}  .top_b{ width:960px; margin:0 auto; line-height:27px;position:relative;}  .top_b_l{ float:left; background:url(images/logo_bg.gif) no-repeat 0 -130px; padding-left:8px;}  .top_b_l a{ padding:0 7px;}  .top_b_r{ float:right; background:url(images/logo_bg.gif) no-repeat 232px -227px; padding-right:19px; height:auto;}  .top_b_r a{ padding:0 12px; float:right;}  #aa{ width:342px ; height:60px; border:solid 5px #000; position:absolute; background:#fff; line-height:60px; text-align:center; z-index:1001; right:0; top:23px; display:none;}

        <div class="top_b_wrap">                <div class="top_b">                         <div class="top_b_l">                                 <a href="#">当前位置:</a><a href="#">首页</a>><a href="#">裸钻</a>                         </div>                                                        <div class="top_b_r">                                 <a href="#" onmouseover="showTopNav('aa')" onmouseout="hideTopNav('aa')">购物车中有0件商品</a><a href="#">注册</a><a href="#">登录</a>                                 <div id="aa">你的购物车中暂无商品</div>                         </div>                  </div>        </div></div>

<script type="text/javascript">function showTopNav(layerName){	eval('document.getElementById('+ "'" + layerName + "'" +').style.display = "block"');}function hideTopNav(layerName){	eval('document.getElementById('+ "'" + layerName + "'" +').style.display = "none"');}</script>