JANK BE GONE!!!

core/modules/navigation/navigation.module

@@ -106,6 +106,15 @@ function navigation_theme($existing, $type, $theme, $path) {
     ],
   ];
 
+  $items['big_pipe_interface_preview__navigation_shortcut_lazy_builder_lazyLinks__Shortcuts'] = [
+    'variables' => [
+      'callback' => NULL,
+      'arguments' => NULL,
+      'preview' => NULL,
+    ],
+    'base hook' => 'big_pipe_interface_preview',
+  ];
+
   $items['block__navigation'] = [
     'render element' => 'elements',
     'base hook' => 'block',

core/modules/navigation/templates/big-pipe-interface-preview--navigation-shortcut-lazy-builder-lazyLinks--Shortcuts.html.twig

+{#
+/**
+ * @file
+ * Placeholder content for the shortcuts menu. This is used to eliminate layout
+ * shifts as BigPipe injects data.
+ */
+#}
+<div class="admin-toolbar__item invisible">
+  <h4 class="toolbar-block__title visually-hidden focusable"></h4>
+  <ul class="toolbar-block__list">
+    <li class="toolbar-block__list-item toolbar-popover">
+      <button class="toolbar-button toolbar-button--icon--shortcuts toolbar-button--expand--side toolbar-button--collapsible toolbar-popover__control" data-index-text="s">
+        <span class="toolbar-button__label ">Shortcuts</span>
+      </button>
+    </li>
+  </ul>
+</div>