Update cancel check to be shadowRoot compatible

This proposal is made to correctly check if the event should be canceled if the first element in the composedPath (or the event.target) matches with the options.cancel selector

ui/widgets/mouse.js

@@ -73,6 +73,11 @@ return $.widget( "ui.mouse", {
 		}
 	},
 
+	/**
+	 *
+	 * @param {MouseEvent & { originalEvent: MouseEvent }} event
+	 * @returns {boolean}
+	 */
 	_mouseDown: function( event ) {
 
 		// don't let more than one widget handle mouseStart
@@ -89,13 +94,15 @@ return $.widget( "ui.mouse", {
 
 		this._mouseDownEvent = event;
 
+		let [ first = event.target ] = 'composedPath' in event.originalEvent && event.originalEvent.composedPath() || []
+
 		var that = this,
 			btnIsLeft = ( event.which === 1 ),
 
 			// event.target.nodeName works around a bug in IE 8 with
 			// disabled inputs (#7620)
-			elIsCancel = ( typeof this.options.cancel === "string" && event.target.nodeName ?
-				$( event.target ).closest( this.options.cancel ).length : false );
+			elIsCancel = ( typeof this.options.cancel === "string" && first.nodeName ?
+				$( first ).closest( this.options.cancel ).length : false );
 		if ( !btnIsLeft || elIsCancel || !this._mouseCapture( event ) ) {
 			return true;
 		}