Get scrollbars dimensions. Taken from http://stackoverflow.com/a/8221501/94746

scrollbars-dimensions.js

function getScrollBarDimensions() {
  var elm = document.documentElement.offsetHeight ? document.documentElement : document.body,
  curX = elm.clientWidth,
  curY = elm.clientHeight,
  hasScrollX = elm.scrollWidth > curX,
  hasScrollY = elm.scrollHeight > curY,
  prev = elm.style.overflow,
  r = { vertical: 0, horizontal: 0 };

  if ( !hasScrollY && !hasScrollX )
    return r;

  elm.style.overflow = "hidden";

  if ( hasScrollY )
    r.vertical = elm.clientWidth - curX;

  if ( hasScrollX )
    r.horizontal = elm.clientHeight - curY;
  
  elm.style.overflow = prev;
  
  return r;
}