Core: provide "includeHidden" parameter in $.ui.scrollParent

Even though the user is unable to scroll via the UI, authors
may have custom scrollbars that programmatically set scrollTop.
Therefore, overflow:hidden can be considered a scrollParent.

ui/core.js

@@ -48,15 +48,16 @@ $.extend( $.ui, {
 
 // plugins
 $.fn.extend({
-	scrollParent: function() {
+	scrollParent: function( includeHidden ) {
 		var position = this.css( "position" ),
 			excludeStaticParent = position === "absolute",
+			overflowRegex = includeHidden ? /(auto|scroll|hidden)/ : /(auto|scroll)/,
 			scrollParent = this.parents().filter( function() {
 				var parent = $( this );
 				if ( excludeStaticParent && parent.css( "position" ) === "static" ) {
 					return false;
 				}
-				return (/(auto|scroll)/).test( parent.css( "overflow" ) + parent.css( "overflow-y" ) + parent.css( "overflow-x" ) );
+				return overflowRegex.test( parent.css( "overflow" ) + parent.css( "overflow-y" ) + parent.css( "overflow-x" ) );
 			}).eq( 0 );
 
 		return position === "fixed" || !scrollParent.length ? $( this[ 0 ].ownerDocument || document ) : scrollParent;