detects the width of the scrollbar. Different depending on OS and browser.

detectScrollbarWidth.js

function detectScrollbarWidth() {
	var outer = document.createElement('div');
	outer.style.visibility = 'hidden';
	outer.style.width = '100px';
	outer.style.msOverflowStyle = 'scrollbar'; // needed for WinJS apps
	
	document.body.appendChild(outer);
	
	var widthNoScroll = outer.offsetWidth;
	// force scrollbars
	outer.style.overflow = 'scroll';
	
	// add innerdiv
	var inner = document.createElement('div');
	inner.style.width = '100%';
	outer.appendChild(inner);
	
	var widthWithScroll = inner.offsetWidth;
	
	// remove divs
	outer.parentNode.removeChild(outer);
	
	scrollbarWidth = widthNoScroll - widthWithScroll;
}