Calculate the width of the browser scrollbars. Credit: http://davidwalsh.name/detect-scrollbar-width

gistfile1.js

var getScrollbarWidth = function() {
	var scrollbarWidth;

	return (function() {
		if (scrollbarWidth) {
			return scrollbarWidth;
		}

		// Create the measurement node
		var scrollDiv = document.createElement("div");

    		// Presumably no browsers have scroll bars wider than 100px!
		scrollDiv.style.width = "100px"
		scrollDiv.style.height = "100px";
		scrollDiv.style.overflow = "scroll";
    
   		// Position it off the screen
		scrollDiv.style.position = "absolute";
		scrollDiv.style.top = "-9999px";

		document.body.appendChild(scrollDiv);

		// Get the scrollbar width
		scrollbarWidth = scrollDiv.offsetWidth - scrollDiv.clientWidth;

		// Delete the DIV 
		document.body.removeChild(scrollDiv);

		return scrollbarWidth;
	})();
}