Ticket #42321: 42321.patch

src/wp-admin/js/nav-menu.js

@@ -829 +829 @@
                                         }
                                 }
                         });
+                        $( '#menu-name' ).on('keyup', _.debounce(function (e) {
+                                menuName = $( document.getElementById( 'menu-name' ) ),
+                            menuNameVal = menuName.val();
+                                if ( !menuNameVal || menuNameVal == menuName.attr('title') || !menuNameVal.replace(/\s+/, '') ) {
+                                        // Add warning for invalid menu name
+                                        menuName.parent().addClass( 'form-invalid' );
+                                } else {
+                                        // Remove warning for valid menu name
+                                        menuName.parent().removeClass( 'form-invalid' );
+                                }
+                        }, 500));
                         $('#add-custom-links input[type="text"]').keypress(function(e){
                                 $('#customlinkdiv').removeClass('form-invalid');
 

src/wp-admin/nav-menus.php

@@ -27 +27 @@
         );
 }
 
+wp_enqueue_script( 'underscore' );
 wp_enqueue_script( 'nav-menu' );
 
 if ( wp_is_mobile() )