Customize: Add notice for when there are no nav menus created yet, prompting user to create one.

Props bpayton, melchoyce, westonruter.
Fixes #42116.


git-svn-id: https://develop.svn.wordpress.org/trunk@41823 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/css/customize-nav-menus.css

@@ -773,6 +773,9 @@ body.adding-menu-items #customize-preview iframe {
 /* @todo update selector */
 #accordion-section-add_menu {
 	margin: 15px 12px;
+}
+
+#accordion-section-add_menu h3 {
 	text-align: right;
 }
 

src/wp-admin/js/customize-nav-menus.js

@@ -1163,7 +1163,8 @@
 		attachEvents: function() {
 			var section = this,
 				container = section.container,
-				contentContainer = section.contentContainer;
+				contentContainer = section.contentContainer,
+				navMenuSettingPattern = /^nav_menu\[/;
 
 			/*
 			 * We have to manually handle section expanded because we do not
@@ -1184,7 +1185,66 @@
 				event.preventDefault();
 			} );
 
-			api.Section.prototype.attachEvents.apply( this, arguments );
+			/**
+			 * Get number of non-deleted nav menus.
+			 *
+			 * @since 4.9.0
+			 * @returns {number} Count.
+			 */
+			function getNavMenuCount() {
+				var count = 0;
+				api.each( function( setting ) {
+					if ( navMenuSettingPattern.test( setting.id ) && false !== setting.get() ) {
+						count += 1;
+					}
+				} );
+				return count;
+			}
+
+			/**
+			 * Update visibility of notice to prompt users to create menus.
+			 *
+			 * @since 4.9.0
+			 * @returns {void}
+			 */
+			function updateNoticeVisibility() {
+				container.find( '.add-new-menu-notice' ).prop( 'hidden', getNavMenuCount() > 0 );
+			}
+
+			/**
+			 * Handle setting addition.
+			 *
+			 * @since 4.9.0
+			 * @param {wp.customize.Setting} setting - Added setting.
+			 * @returns {void}
+			 */
+			function addChangeEventListener( setting ) {
+				if ( navMenuSettingPattern.test( setting.id ) ) {
+					setting.bind( updateNoticeVisibility );
+					updateNoticeVisibility();
+				}
+			}
+
+			/**
+			 * Handle setting removal.
+			 *
+			 * @since 4.9.0
+			 * @param {wp.customize.Setting} setting - Removed setting.
+			 * @returns {void}
+			 */
+			function removeChangeEventListener( setting ) {
+				if ( navMenuSettingPattern.test( setting.id ) ) {
+					setting.unbind( updateNoticeVisibility );
+					updateNoticeVisibility();
+				}
+			}
+
+			api.each( addChangeEventListener );
+			api.bind( 'add', addChangeEventListener );
+			api.bind( 'removed', removeChangeEventListener );
+			updateNoticeVisibility();
+
+			api.Section.prototype.attachEvents.apply( section, arguments );
 		},
 
 		/**