Core: Allow shadow DOM in attachment check

Allow isAttached to check shadow DOM for attachment. This change allows
isAttached() function to address shadowDOM elements even if the browser doesn't support getRootNode()

Ref jquerygh-3504, jquerygh-3977

src/var/getShadowRoot.js

@@ -0,0 +1,22 @@
+define( [
+	"./document"
+], function( document ) {
+	"use strict";
+
+	return function getShadowRoot( elem ) {
+
+		// Check if browser supports Shadow DOM
+		if ( document.head.createShadowRoot || document.head.attachShadow ) {
+
+			// check for support of getRootNode function
+			while ( typeof elem.getRootNode !== "function" ) {
+				var doc = elem.ownerDocument;
+				elem = doc && doc.host ? doc.host : doc;
+			}
+			return elem.getRootNode( { composed: true } );
+		}
+
+		return false;
+	};
+
+} );

src/var/isAttached.js

@@ -1,11 +1,14 @@
 define( [
 	"../core",
+	"./getShadowRoot",
 	"../selector" // Get jQuery.contains
-], function( jQuery ) {
+], function( jQuery, getShadowRoot ) {
 	"use strict";
 
-	return function isAttached( obj ) {
-		return jQuery.contains( obj.ownerDocument, obj );
-	};
+	// Replace the use of contains function to check attachment (gh-3504)
+	return function isAttached( elem ) {
 
+		return jQuery.contains( elem.ownerDocument, elem ) ||
+			getShadowRoot( elem ) === elem.ownerDocument;
+	};
 } );