Scratch

$(window).scroll(function () {

 if (window.innerHeight + window.pageYOffset < 1020) {
  document.getElementsByTagName('body')[0].style.backgroundAttachment = "scroll";
  document.getElementsByTagName('body')[0].style.backgroundPosition = "0% 0%"
 }
 if (window.innerHeight + window.pageYOffset > 1021) {
  if (window.backgroundExtra === undefined) { window.backgroundExtra = 1021- window.innerHeight; }
  document.getElementsByTagName('body')[0].style.backgroundAttachment = "fixed";
  document.getElementsByTagName('body')[0].style.backgroundPosition = "0% -" + window.backgroundExtra + "px"
 }
});