Customizer: Ensure that all existing menus are shown in the Custom Menu widget's dropdown.

* Ensure that a Custom Menu widget selecting a newly-inserted menu gets updated to use the new menu ID upon Save & Publish.
* Dynamically update the visibility of the Custom Menu widget's "no menus" message when the number of menus changes between 0 and 1+.
* Send all dirty Customized settings in `update-widget` Ajax request and `preview()` them so that the widget update/form callbacks have access to any data dependencies in the current Customizer session (such as newly created unsaved menus).
* Update link in Custom Menu widget to point to Menus panel as opposed to Menus admin page, when in the Customizer.
* Fix an issue with extra space at top immediately after creating new menu.
* Fix doubled `update-widget` Ajax requests when changing select dropdown; prevent initial from being aborted.
* Add missing `wp_get_nav_menus()` hooks to preview Customizer updates/inserts for `nav_menu` settings; includes tests.
* Update `wp_get_nav_menu_object()` to allow a menu object to be passed in (and thus passed through).

Props westonruter, adamsilverstein.
Fixes #32814.


git-svn-id: https://develop.svn.wordpress.org/trunk@33488 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/nav-menu.php

@@ -12,12 +12,17 @@
  *
  * @since 3.0.0
  *
- * @param string $menu Menu ID, slug, or name.
+ * @param string $menu Menu ID, slug, or name - or the menu object.
  * @return object|false False if $menu param isn't supplied or term does not exist, menu object if successful.
  */
 function wp_get_nav_menu_object( $menu ) {
 	$menu_obj = false;
-	if ( $menu ) {
+
+	if ( is_object( $menu ) ) {
+		$menu_obj = $menu;
+	}
+
+	if ( $menu && ! $menu_obj ) {
 		$menu_obj = get_term( $menu, 'nav_menu' );
 
 		if ( ! $menu_obj ) {