// ==UserScript==
// @name         9gag - Scroll down to current position
// @namespace    https://github.com/Procyon-b
// @version      0.3
// @description  Scroll down back where the focus was when you clicked the arrow-up icon or scrolled up
// @author       Achernar
// @match        https://9gag.com/*
// @grant        none
// @downloadURL none
// ==/UserScript==

(function() {
"use strict";

var pos=[0,0], allPos={}, L, btt=document.querySelector('.back-to-top')

if (!btt) return;

window.addEventListener('scroll', savePos);

function savePos() {
  var d=window.scrollY - pos[1];
  if (d < 0) showHide(true);
  if (d >= 0) {
    pos[0]=window.scrollX;
    pos[1]=window.scrollY;
    showHide();
    }
  }

function showHide(show=false) {
  if (show == L.disp) return;
  back.style.display= show?'':'none';
  L.disp=show;
  }

var back=document.createElement('a');

document.querySelector('header#top-nav').appendChild(back);

back.href='javascript:;';
back.classList='back-to-top';
back.style='transform: rotate(180deg); top: 5em; display: none;';
back.title='Scroll back to position';

back.addEventListener('click', function(e){
  if (pos) window.scrollTo.apply(null, pos);
  showHide();
  });

var hRS=history.replaceState;

history.replaceState=function() {
  setTimeout(LocPos, 0);
  // unlink "pos" "L"
  pos=[0,0];
  L={disp:false};
  return hRS.apply(history, arguments);
  }

function LocPos() {
  if (!allPos[location.href]) allPos[location.href]={pos:[0,0], disp:false};
  L=allPos[location.href];
  pos=L.pos;
  back.style.display= L.disp ? '':'none';
  }

LocPos();

})();