core: fixed #2993: .offsetParent() will now return the body element i…

…n all browsers for fixed position elements.

src/offset.js

@@ -132,7 +132,7 @@ jQuery.fn.extend({
 	},
 
 	offsetParent: function() {
-		var offsetParent = this[0].offsetParent;
+		var offsetParent = this[0].offsetParent || document.body;
 		while ( offsetParent && (!/^body|html$/i.test(offsetParent.tagName) && jQuery.css(offsetParent, 'position') == 'static') )
 			offsetParent = offsetParent.offsetParent;
 		return jQuery(offsetParent);