Untitled

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>JS Bin</title>
<style id="jsbin-css">
html, body {
  margin: 0;
  padding: 0;
}

.main {
  height: 2000px;
  background: linear-gradient(to top, yellow, purple)
}

.bar {
  position: fixed;
  width: 5px;
  background: red;
  min-height: 15%;
  right: 0;
  top: 0;
}
</style>
</head>
<body>
<div class="bar">
</div>
  <div class="main"></div>
<script id="jsbin-javascript">
var $main = document.querySelector('.main')
var height = $main.offsetHeight
var wHeight = window.innerHeight

var barHeight = wHeight / height * wHeight
var $bar = document.querySelector('.bar')
$bar.style.height = barHeight + 'px'
barHeight = $bar.offsetHeight

var $body = document.querySelector('body')
window.addEventListener('scroll', function () {
  var total = (wHeight - barHeight) / barHeight
  var progress = $body.scrollTop / (height - wHeight)
  var translate = progress * total * 100
  $bar.style.transform = 'translateY(' + translate + '%)'
})
</script>


<script id="jsbin-source-css" type="text/css">html, body {
  margin: 0;
  padding: 0;
}

.main {
  height: 2000px;
  background: linear-gradient(to top, yellow, purple)
}

.bar {
  position: fixed;
  width: 5px;
  background: red;
  min-height: 15%;
  right: 0;
  top: 0;
}</script>

<script id="jsbin-source-javascript" type="text/javascript">var $main = document.querySelector('.main')
var height = $main.offsetHeight
var wHeight = window.innerHeight

var barHeight = wHeight / height * wHeight
var $bar = document.querySelector('.bar')
$bar.style.height = barHeight + 'px'
barHeight = $bar.offsetHeight

var $body = document.querySelector('body')
window.addEventListener('scroll', function () {
  var total = (wHeight - barHeight) / barHeight
  var progress = $body.scrollTop / (height - wHeight)
  var translate = progress * total * 100
  $bar.style.transform = 'translateY(' + translate + '%)'
})
</script></body>
</html>