Use a placeholder for the navigation render menu.

Closes #3498922

core/modules/navigation/js/user-block.js

-/**
- * @file
- *
- * Customizes the user block to use the current username without affecting
- * caching.
- */
-
-((Drupal, drupalSettings, once) => {
-  /**
-   * Replaces the generic 'My Account' text with the actual username.
-   *
-   * @type {Drupal~behavior}
-   *
-   * @prop {Drupal~behaviorAttach} attach
-   *   Sets the username.
-   */
-  Drupal.behaviors.navigationUsername = {
-    attach: (context, settings) => {
-      if (!settings?.navigation?.user) {
-        return;
-      }
-      once('user-block', '[data-user-block]', context).forEach((userBlock) => {
-        userBlock
-          .querySelectorAll(
-            '.toolbar-popover__control, .toolbar-popover__header',
-          )
-          .forEach((button) => {
-            const buttonLabel = button.querySelector('[data-toolbar-text]');
-            button.dataset.indexText = settings.navigation.user.charAt(0);
-            button.dataset.iconText = settings.navigation.user.substring(0, 2);
-            if (buttonLabel) {
-              buttonLabel.textContent = settings.navigation.user;
-            }
-          });
-      });
-    },
-  };
-})(Drupal, drupalSettings, once);