Sortable: Improved compatibility with flexbox

Added detection for flexbox into _isFloating, which improves the
detection of where an item is moved to

Ref: #9344

Author: CBenni

ui/widgets/sortable.js

@@ -82,7 +82,7 @@ return $.widget( "ui.sortable", $.ui.mouse, {
 	},
 
 	_isFloating: function( item ) {
-		return ( /left|right/ ).test( item.css( "float" ) ) || ( /inline|table-cell/ ).test( item.css( "display" ) );
+		return ( /left|right/ ).test( item.css( "float" ) ) || ( /inline|table-cell/ ).test( item.css( "display" ) ) || ( /flex/ ).test( this.element.css( "display" ) );
 	},
 
 	_create: function() {