Commonwealth Stadium - Alternative Configuration, 1973 map w/ slider

<!DOCTYPE html>
<html>
<head>
<meta charset=utf-8 />
<title>Commonwealth Stadium - Alternative Configuration, 1973</title>
<meta name='viewport' content='initial-scale=1,maximum-scale=1,user-scalable=no' />
<script src='https://api.mapbox.com/mapbox.js/v2.2.2/mapbox.js'></script>
<link href='https://api.mapbox.com/mapbox.js/v2.2.2/mapbox.css' rel='stylesheet' />
<style>
  body { margin:0; padding:0; }
  #map { position:absolute; top:0; bottom:0; width:100%; }
</style>
</head>
<body>
<style>
.title {
  position: absolute;
  right: 20px;
  top: 10px;
  font-family: "Proxima Nova", Montserrat, sans-serif;
  text-align: right;
  max-width: 600px;
  z-index: 10001;
}
h1, h2 {    
  font-weight: 100;
  color: #fff; /* offical UK Kentucky blue */
  line-height: 5px;
  text-shadow: 0 0 0.2em #000;
}
h1 {
  font-size: 2em;
}
h2 {
  font-size: 1em;
}

.ui-opacity {
  background:#FFF;
  position:absolute;
  left:10px;
  top:70px;
  height:200px;
  width:28px;
  border:1px solid rgba(0,0,0,0.4);
  border-radius:3px;
  z-index:1000;
  }
.ui-opacity .handle {
  position:absolute;
  background:#404040;
  left:0;
  top:20px;
  width:26px;
  height:10px;
  border-radius:1px;
  cursor:pointer;
  cursor:ns-resize;
  }
  .ui-opacity .handle:hover {
    background:#303030;
    }
</style>
<div class="title">
  <h1>Commonwealth Stadium</h1>
  <h2> Alternative Configuration, 1973</h2>
</div>
<div id='map'></div>
<div id='control' class='ui-opacity'>
  <div id='handle' class='handle'></div>
</div>

var map = L.mapbox.map('map', 'maptastik.nc2c5lfc',{
      minZoom: 14,
      maxZoom: 19
    })
    .setView([38.021800, -84.509486], 16);

var overlay = L.tileLayer('https://mapwarper.net/maps/tile/10571/{z}/{x}/{y}.png', {
      zIndex: 2,
      attribution: '<a href="https://www.kyphotoarchive.com/2015/09/04/plans-for-roads-around-commonwealth-stadium-1973/" target="_blank">Kentucky Photo Archive</a>, <a href="https://mapwarper.net/maps/10571" target="_blank">Map Warper</a>'
    })
    .addTo(map);

document.onmousemove = function(e) {
    if (!start) return;
    // Adjust control.
    handle.style.top = Math.max(-5, Math.min(195, startTop + parseInt(e.clientY, 10) - start)) + 'px';
    // Adjust opacity.
    overlay.setOpacity(1 - (handle.offsetTop / 200));
};

handle.onmousedown = function(e) {
    // Record initial positions.
    start = parseInt(e.clientY, 10);
    startTop = handle.offsetTop - 5;
    return false;
};

document.onmouseup = function(e) {
    start = null;
};
</script>
</body>
</html>

https://api.mapbox.com/mapbox.js/v2.2.2/mapbox.js