add common styles.

src/components.css

@@ -60,7 +60,7 @@
     align-items: center;
   }
 
-  .spacer {
+  :where(.spacer, .extendable) {
     flex: 1 1;
   }
 
@@ -467,6 +467,9 @@
     flex-direction: row;
     align-items: center;
     gap: calc(var(--spacing) * 2);
+    border-bottom: 1px solid var(--color-on-surface-variant);
+    border-top-left-radius: calc(var(--border-radius) * 2);
+    border-top-right-radius: calc(var(--border-radius) * 2);
     background-color: var(--color-surface-container-highest);
   }
 }