弹出框背景色透明滚动条滚动仍然居中

弹出框div：
<div class="webbox" style="">
            <div class="webboxtop">
                <span class="tstitle">发货网点（收货网点）</span>
            <a href="#" class="closebox"></a>
            </div>
            <div class="webboxnr">
                <input type="text" class="search" id="tt" value="" onblur="this.focus();" style="">
            </div>
        </div>

背景透明div
<div class="zzblack" style="display: block;"></div>

居中css：
.webbox{ width: 520px;height: 350px; background: #FFFFFF;position: fixed; z-index: 999; left: 50%; top: 50%; margin-left: -260px; margin-top: -150px;}
.webboxtop{ height: 40px; line-height: 40px; background: #00C049;  padding-left: 15px; color: #FFFFFF; font-size: 14px; }
.closebox{ display: block;  float:right;width: 28px; height: 28px; background: url(../img/close.png) no-repeat; margin-top: 6px; margin-right: 10px;}
.tstitle{ float: left; display: block;}
.webboxnr{ width: 450px; margin: 0 auto; line-height: 30px; margin-top: 20px; }
.search{ width: 450px; height: 35px; line-height: 35px; color: #666666; font-size: 14px; text-indent:0.5em;border: 1px solid #CCCCCC; border-radius: 5px;}
.zzblack{ width:100%; height:100%;position: absolute; top: 0; left: 0; background: #888; opacity: 0.5;-webkit-opacity: 0.5; -moz-opacity: 0.5;filter: progid:DXImageTransform.Microsoft.Alpha(opacity=50);}