scroll original window -> new window will be scrolled synchronously

scroll-sync.js

//javascript:
(() => {
  const mkDebounce = fn => {
    let timeoutId;
    let resolves = [];
    return function(...args) {
      return new Promise(resolve => {
        clearTimeout(timeoutId);
        timeoutId = setTimeout(() => {
          timeoutId = null;
          for (r of resolves) r(fn.apply(this, args));
          resolves = [];
        }, 250);
        resolves.push(resolve);
      });
    };
  };

  const state = {
    sync: true,
    offset: {
      X: 0,
      Y: 0
    }
  };

  const sideUrl = window.prompt(
    "Same origin URL to mirror",
    window.location.href
  );
  if (!sideUrl) {
    return;
  }
  Notification.requestPermission(p => {
    const msgs = [
      "You can change offset",
      "by calling _updateOffset() in DevTools"
    ];
    if (p === "granted")
      new Notification(msgs[0], {
        body: msgs[1]
      });
    else console.log(msgs.join(" "));
  });

  const sideWindow = window.open(sideUrl);

  function updateOffset() {
    state.sync = !state.sync;
    state.offset.X = sideWindow.scrollX - window.scrollX;
    state.offset.Y = sideWindow.scrollY - window.scrollY;
  }
  window._updateOffset = updateOffset;

  const onScroll = mkDebounce(() => {
    if (state.sync) {
      sideWindow.scrollTo(
        window.scrollX + state.offset.X,
        window.scrollY + state.offset.Y
      );
    }
  });

  document.addEventListener("scroll", onScroll);
})();