/* max-height: 100% is key */
.overlay{background: url(../images/bk_cube.png);z-index: 9999;width: 100%;max-height: 100%;text-align: center;position: fixed; overflow: auto; top:0; left: 0;overflow-y: scroll; display: none;}

/* position: static; is key */
#app_detail > .wrapper{width: 1120px; background: #FFF; margin: 0 auto; padding:20px; position: static;}

function show_detail(){
  $('body').css({'overflow':'hidden'});
  $('#app_detail').show();
  init_carousel();
  init_gallery();
}

http://stackoverflow.com/questions/18894400/fixed-position-div-scrollable