gistfile1.js

getViewportScrollY = function() {
  var scrollY = 0;
  if( document.documentElement && document.documentElement.scrollTop ) {
    scrollY = document.documentElement.scrollTop;
  }
  else if( document.body && document.body.scrollTop ) {
    scrollY = document.body.scrollTop;
  }
  else if( window.pageYOffset ) {
    scrollY = window.pageYOffset;
  }
  else if( window.scrollY ) {
    scrollY = window.scrollY;
  }
  return scrollY;
};