Page Scroll Marker

Adds translucent bars to the top and bottom of the viewport, which indicates the previous viewport position when the page is scrolled. This tool will prevents you from getting lost when you press the Page Up/Down keys.
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// ==UserScript==
// @name	Page Scroll Marker
// @namespace	http://userscripts.org/users/mstm
// @description	Adds translucent bars to the top and bottom of the viewport, which indicates the previous viewport position when the page is scrolled. This tool will prevents you from getting lost when you press the Page Up/Down keys.
// @version	0.9.2
// @include	*
// @grant	GM_addStyle
// @grant	GM_getValue
// @grant	GM_setValue
// ==/UserScript==

(function () {
  if (document.designMode == 'on' || document.body instanceof HTMLFrameSetElement || document.URL.indexOf(location.protocol) !== 0) return;

  var generalName = GM_info.script.name;
  var containerID = GM_info.script.namespace.concat(generalName).replace(/\W/g, '');

  GM_addStyle('.' + containerID + ' { display: none !important; background: transparent !important; border: none !important; } .' + containerID + '.active { display: block !important; } .' + containerID + ' div { position: absolute !important; width: 100% !important; height: 8px !important; background: #808080 !important; z-index: 65535 !important; opacity: 0.25 !important; font: 8px/1 sans-serif !important; color: #FF0000 !important; text-align: center !important; border: none !important; } .' + containerID + ' div:after { content: attr(title); }');

  var ScrollNode = function (target) {
    this.initialize(target);
  };

  ScrollNode.prototype = {
    update: function () {
      clearTimeout(this.tid);
      this.lines.className = this.lines.className.replace(/\bactive\b/g, '').replace(/\s+/, ' ').replace(/^\s?(.*?)\s?$/, '$1');
      this.upper.style.top = + this.owner.scrollTop + 'px';
      this.lower.style.bottom = - this.owner.scrollTop + 'px';
    },

    scroll: function () {
      clearTimeout(this.tid);
      this.upper.style.left = this.lower.style.left = this.owner.scrollLeft + 'px';
      this.lines.className = this.lines.className.split(' ').concat('active').join(' ');
      this.tid = setTimeout((function (o, f) {
        return function () {
          f.call(o);
        };
      })(this, this.update), 1000);

      if (GM_getValue('wrap_bars', true)) {
        if (this.lower.offsetTop + this.lower.offsetHeight < this.owner.scrollTop) {
          this.upper.style.top = parseInt(this.upper.style.top) + this.owner.clientHeight + 'px';
          this.lower.style.bottom = parseInt(this.lower.style.bottom) - this.owner.clientHeight + 'px';
        }

        if (this.upper.offsetTop - this.owner.clientHeight > this.owner.scrollTop) {
          this.upper.style.top = parseInt(this.upper.style.top) - this.owner.clientHeight + 'px';
          this.lower.style.bottom = parseInt(this.lower.style.bottom) + this.owner.clientHeight + 'px';
        }
      }
    },

    initialize: function (target) {
      this.isTop = target.nodeName == document.nodeName;
      this.owner = this.isTop ? target.body : target;
      this.lines = this.owner.appendChild(document.createElement('DIV'));
      this.lines.className = containerID;
      this.lines.title = generalName;
      this.upper = this.lines.appendChild(document.createElement('DIV'));
      this.upper.title = '\u25bc';
      this.lower = this.lines.appendChild(document.createElement('DIV'));
      this.lower.title = '\u25b2';

      if (this.isTop) {
        if (document.compatMode != 'BackCompat') {
          this.owner = document.documentElement;
        }
      }
      else {
        if (document.defaultView.getComputedStyle(this.owner, null).getPropertyValue('position') == 'static') {
          this.owner.style.position = 'relative';
        }

        this.lines.addEventListener('DOMNodeRemoved', function (e) {
          ScrollNode.put(e.relatedNode);
        }, false);

        this.lines.addEventListener('DOMNodeRemoved', function (e) {
          e.stopPropagation();
        }, true);
      }

      this.update();
    }
  };

  ScrollNode.instances = {};
  ScrollNode.remaining = 10;

  ScrollNode.put = function (target) {
    if (--this.remaining < 0) {
      return this.get(document);
    }

    var s = new ScrollNode(target);
    this.instances[s.isTop ? target.nodeName : (target.id = target.id || '__id__' + (this.uid = (this.uid || 0) + 1))] = s;
    return s;
  };

  ScrollNode.get = function (target) {
    return this.instances[target.nodeName] || this.instances[target.id];
  };

  ScrollNode.put(document);

  document.addEventListener('scroll', function (e) {
    if (GM_getValue('recursive', true) && !('value' in e.target)) {
      (ScrollNode.get(e.target) || ScrollNode.put(e.target)).scroll();
    }
  }, true);

  document.addEventListener('resize', function (e) {
    for (var sf in ScrollNode.instances) {
      ScrollNode.instances[sf].update();
    }
  }, true);

  GM_setValue('wrap_bars', GM_getValue('wrap_bars', true));
  GM_setValue('recursive', GM_getValue('recursive', true));
})()
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