gistfile1.js

var current = 0;
var target = 0;
var updateDelay = 0;
var speed = 30 / 1000;
var lastTime = performance.now();


function step() {
  requestAnimationFrame(step);

  var time = performance.now();
  var sinceLastUpdate = (time - lastTime) / 1000;
  lastTime = time;
  
  if (updateDelay > 0) {
    updateDelay = Math.max(-speed, updateDelay - sinceLastUpdate);
  }
  
  if (current === target) return;
  
  var direction = current < target ? +1 : -1;
  while (current !== target && updateDelay <= 0) {
    current += direction;
    updateDelay += speed;
  }
}

window.onscroll = function() {
  target = window.scrollY; // ?
};


step();