fix back button for SPA example (fixes #3910)

examples/single_page_app/router_frame.js

@@ -1,10 +1,9 @@
 export default {
   template: "<div><slot></slot></div>",
   mounted() {
+    const initial_path = window.location.pathname;
     window.addEventListener("popstate", (event) => {
-      if (event.state?.page) {
-        this.$emit("open", event.state.page);
-      }
+      this.$emit("open", event.state?.page || initial_path);
     });
     const connectInterval = setInterval(async () => {
       if (window.socket.id === undefined) return;