remember-page-scroll-position.user.js

// ==UserScript==
// @name         Remember page scroll position
// @namespace    https://greasyfork.org/en/users/85671-jcunews
// @version      1.0.1
// @license      AGPL v3
// @author       jcunews
// @description  Context: https://www.reddit.com/r/userscripts/comments/1ayfnoh/add_scroll_position_to_url_is_this_possible/
// @match        *://*/*
// @include      *:*
// @grant        GM_getValue
// @grant        GM_setValue
// ==/UserScript==

//Note: Scroll position applies to page only. Not elements within page. i.e. doesn't apply to scroll position of element content. Only the page.

(() => {
  //=== CONFIG BEGIN
  var delayBeforeApplyPagePosition = 500; //in milliseconds. 1 second = 1000ms. since last change to page height and position.
  var delayBeforeSavePagePosition  = 500; //in milliseconds. 1 second = 1000ms. since last change to page position.
  var maxDaysToKeepSavedPositions  = 30;  //since last update to saved page position.
  //=== CONFIG END

  var pageHeight, posX, posY, timestamp, timer;
  function savePos(recs, rec) {
    recs = GM_getValue("records", {});
    rec = recs[location.href] = recs[location.href] || {};
    rec.x = scrollX;
    rec.y = scrollY;
    rec.ts = Date.now();
    Object.keys(recs).forEach(k => {
      if ((Date.now() - recs[k].ts) >= maxDaysToKeepSavedPositions*24*60*60*1000) delete recs[k]
    });
    GM_setValue("records", recs)
  }
  pageHeight = document.body.scrollHeight;
  posX = scrollX;
  posY = scrollY;
  timestamp = Date.now();
  (function checkPage(rec) {
    if ((document.body.scrollHeight !== pageHeight) || (document.body.scrollLeft !== scrollX) || (document.body.scrollTop !== scrollY)) {
      pageHeight = document.body.scrollHeight;
      posX = scrollX;
      posY = scrollY;
      timestamp = Date.now()
    } else if ((Date.now() - timestamp) >= delayBeforeApplyPagePosition) {
      addEventListener("beforeunload", savePos);
      addEventListener("blur", savePos);
      addEventListener("focus", savePos);
      addEventListener("scroll", () => {
        clearTimeout(timer);
        timer = setTimeout(savePos, delayBeforeSavePagePosition)
      });
      if (rec = GM_getValue("records", {})[location.href]) scrollTo(rec.x, rec.y);
      return
    }
    setTimeout(checkPage, 100)
  })()
})()