Issue #2951268 by dww, ndf, Chi, bnjmnm, lauriii, drintios, kishor_kolekar, tim.plunkett, quietone: Improve rendering account link in the toolbar

core/lib/Drupal/Core/Render/Renderer.php

@@ -327,6 +327,7 @@ class Renderer implements RendererInterface {
         '#cache',
         '#create_placeholder',
         '#lazy_builder_preview',
+        '#preview',
         // The keys below are not actually supported, but these are added
         // automatically by the Renderer. Adding them as though they are
         // supported allows us to avoid throwing an exception 100% of the time.

core/modules/user/user.module

@@ -1181,6 +1181,12 @@ function user_toolbar() {
     $items['user']['tab']['#title'] = [
       '#lazy_builder' => ['user.toolbar_link_builder:renderDisplayName', []],
       '#create_placeholder' => TRUE,
+      '#lazy_builder_preview' => [
+        // Add a line of whitespace to the placeholder to ensure the icon is
+        // positioned in the same place it will be when the lazy loaded content
+        // appears.
+        '#markup' => ' ',
+      ],
     ];
     $items['user']['tray']['user_links'] = [
       '#lazy_builder' => ['user.toolbar_link_builder:renderToolbarLinks', []],