// related to this? https://bugs.webkit.org/show_bug.cgi?id=236312

/* 
On iOS Safari, scroll values get reverted in odd ways. It's almost like when we window.scrollTo(...), 
it does an async "scroll" event dispatch so the values might be stale. For example, if we window.scrollTo(0, 100)
and then window.scrollTo(0, 200) on the very next touchmove event and perhaps the requestAnimationFrame() hadn't
fired yet, our "scroll" event handler will be called later, first with a scroll position of 100 (even though we 
set it to 200 BEFORE this event fires!!!) and then again at 200. So the order goes like this:

- set scroll 100
- set scroll 200
- "scroll" event handler fires, and the pageYOffset is 100
- "scroll" event handler fires, and the pageYOffset is 200

They can actually get even more out of order, like 3 or 4 deep. 
*/

let positions = [window.pageYOffset],
    scroll, startY, recent;

document.addEventListener("touchstart", e => {
  startY = e.changedTouches[0].clientY;
  scroll = recent = window.pageYOffset;
  e.preventDefault();
}, { passive: false });

document.addEventListener("touchmove", e => {
  recent = scroll + (startY - e.changedTouches[0].clientY);
  positions.push(recent);
  window.scrollTo(0, recent);
  e.preventDefault();
}, { passive: false });

document.addEventListener("touchend", e => positions = [window.pageYOffset]);

// on scroll, make sure the scroll position matches what we set it to most recently. This is where the problem shows up. 
function onScroll(e) {
  if (window.pageYOffset !== recent) {
    log("PROBLEM! Scroll is " + window.pageYOffset + " but should be: " + recent + ". Last few: " + positions.slice(Math.max(0, positions.length-4)).join(", "));
  }
}

document.addEventListener("scroll", onScroll, { passive: true });


function log(message) {
  document.querySelector(".log").innerHTML += message + "<br>";
}

document.querySelectorAll("section").forEach((el, i) => el.style.backgroundImage = `url(https://picsum.photos/800/600?random=${i})`);

// more: https://codepen.io/GreenSock/pen/qBVgwqO/59bdd603bc25a229dadf4fb88117d309