Core: Recognize Shadow DOM in attachment checks

Allow `isAttached` to check Shadow DOM for attachment.

Fixes gh-3504
Closes gh-3996
Ref gh-3977

1 files changed, 22 insertions, 0 deletions

diff --git a/src/core/isAttached.js b/src/core/isAttached.js
new file mode 100644
index 000000000..efa2465a9
--- /dev/null
+++ b/src/core/isAttached.js
@@ -0,0 +1,22 @@
+define( [
+	"../core",
+	"../var/documentElement",
+	"../selector" // jQuery.contains
+], function( jQuery, documentElement ) {
+	"use strict";
+
+	var isAttached = function( elem ) {
+			return jQuery.contains( elem.ownerDocument, elem );
+		},
+		composed = { composed: true };
+
+	// Check attachment across shadow DOM boundaries when possible (gh-3504)
+	if ( documentElement.attachShadow ) {
+		isAttached = function( elem ) {
+			return jQuery.contains( elem.ownerDocument, elem ) ||
+				elem.getRootNode( composed ) === elem.ownerDocument;
+		};
+	}
+
+	return isAttached;
+} );