Fix expand animation

Closes #3402067

css/components/toolbar-button.css

@@ -190,3 +190,9 @@ html:not([data-admin-toolbar="expanded"]) .admin-toolbar .toolbar-button__label
   clip-path: inset(50%);
   opacity: 0;
 }
+
+/* Hides button's label while transitioning. */
+
+html[data-admin-toolbar-animating][data-admin-toolbar="expanded"] .admin-toolbar .toolbar-button__label {
+  display: none;
+}