.masklayer{width: 100%; height: 100%; position: fixed; background: rgba(0,0,0,0.6); overflow: auto;}
.popup{min-height: 100%;}
.popup .main{padding-bottom: 1rem;}
.close{padding-bottom: .2rem; margin-top: -1rem; position: relative; clear: both; text-align: center;}
<div class="masklayer">
<div class="popup clearfix">
<div class="main">
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</div>
</div>
<div class="close">close</div>
</div>