Add focus button.

src/style.css

@@ -219,6 +219,17 @@
   border-color: #5b4d80;
 }
 
+/* The pane currently filling its tab. The other panes are still open and still
+   running, just not drawn, so this is marked quietly — a lit border rather than
+   anything alarming. The header's toggle icon carries the rest of the message. */
+.playpen-pane.zoomed {
+  border-color: #7a68a8;
+}
+
+.playpen-pane.zoomed .playpen-pane-button {
+  opacity: 0.55;
+}
+
 /* The pane being dragged. There is no separate drop indicator: the layout
    rearranges live during the drag, so the view itself is the preview. */
 .playpen-pane.dragging {