Issue #3484564: Define the 3 areas the Top Bar will provide

Closes #3484564

core/modules/navigation/css/components/admin-toolbar.css

@@ -63,7 +63,7 @@ body {
   z-index: var(--admin-toolbar-z-index);
   display: flex;
   flex-direction: column;
-  height: 100vh;
+  block-size: 100vh;
   transform: translateX(-100%);
   background-color: var(--admin-toolbar-color-white);
   font-family: var(--admin-toolbar-font-family);
@@ -92,7 +92,9 @@ body {
 }
 @media (min-width: 64rem) {
   .admin-toolbar {
+    block-size: calc(100vh - var(--drupal-displace-offset-top, 0px));
     transform: none;
+    inset-block-start: var(--drupal-displace-offset-top, 0);
   }
 }
 @media only screen and (max-height: 18.75rem) {