$("#foo", xml) would always return an empty set, fixed (bug #877). Ad…

…ditionally, a bug in jQuery.isXMLDoc(xmlDoc) was discovered, if the element was, in fact, an XML document.

src/jquery/jquery.js

@@ -1318,7 +1318,8 @@ jQuery.extend({
 
 	// check if an element is in a XML document
 	isXMLDoc: function(elem) {
-		return elem.tagName && elem.ownerDocument && !elem.ownerDocument.body;
+		return elem.documentElement && !elem.body ||
+			elem.tagName && elem.ownerDocument && !elem.ownerDocument.body;
 	},
 
 	nodeName: function( elem, name ) {

src/selector/selector.js

@@ -213,7 +213,7 @@ jQuery.extend({
 					var elem = ret[ret.length-1];
 
 					// Try to do a global search by ID, where we can
-					if ( m[1] == "#" && elem && elem.getElementById ) {
+					if ( m[1] == "#" && elem && elem.getElementById && !jQuery.isXMLDoc(elem) ) {
 						// Optimization for HTML document case
 						var oid = elem.getElementById(m[2]);