Allow :focus selector to pass through QSA. Breaks Chrome 21, bypasses…

… IE8/9 throwing an error. Supplements jQuery #13378.

dist/sizzle.js

@@ -6,7 +6,7 @@
  * Released under the MIT license
  * http://jquery.org/license
  *
- * Date: 2013-04-04
+ * Date: 2013-04-08
  */
 (function( window, undefined ) {
 
@@ -287,7 +287,7 @@ function Sizzle( selector, context, results, seed ) {
 		}
 
 		// QSA path
-		if ( support.qsa && !rbuggyQSA.test(selector) ) {
+		if ( support.qsa && (!rbuggyQSA || !rbuggyQSA.test( selector )) ) {
 			nid = old = expando;
 			newContext = context;
 			newSelector = nodeType === 9 && selector;
@@ -504,10 +504,12 @@ setDocument = Sizzle.setDocument = function( node ) {
 	// matchesSelector(:active) reports false when true (IE9/Opera 11.5)
 	rbuggyMatches = [];
 
-	// qSa(:focus) reports false when true (Chrome 21),
-	// no need to also add to buggyMatches since matches checks buggyQSA
-	// A support test would require too much code (would include document ready)
-	rbuggyQSA = [ ":focus" ];
+	// qSa(:focus) reports false when true (Chrome 21)
+	// We allow this because of a bug in IE8/9 that throws an error
+	// whenever `document.activeElement` is accessed on an iframe
+	// So, we allow :focus to pass through QSA all the time to avoid the IE error
+	// See http://bugs.jquery.com/ticket/13378
+	rbuggyQSA = [];
 
 	if ( (support.qsa = isNative(doc.querySelectorAll)) ) {
 		// Build QSA regex
@@ -578,7 +580,7 @@ setDocument = Sizzle.setDocument = function( node ) {
 		});
 	}
 
-	rbuggyQSA = new RegExp( rbuggyQSA.join("|") );
+	rbuggyQSA = rbuggyQSA.length && new RegExp( rbuggyQSA.join("|") );
 	rbuggyMatches = rbuggyMatches.length && new RegExp( rbuggyMatches.join("|") );
 
 	// Element contains another
@@ -712,7 +714,10 @@ Sizzle.matchesSelector = function( elem, expr ) {
 	expr = expr.replace( rattributeQuotes, "='$1']" );
 
 	// rbuggyQSA always contains :focus, so no need for an existence check
-	if ( support.matchesSelector && documentIsHTML && (!rbuggyMatches || !rbuggyMatches.test(expr)) && !rbuggyQSA.test(expr) ) {
+	if ( support.matchesSelector && documentIsHTML &&
+		(!rbuggyMatches || !rbuggyMatches.test(expr)) &&
+		(!rbuggyQSA     || !rbuggyQSA.test(expr)) ) {
+
 		try {
 			var ret = matches.call( elem, expr );