Fix Router navigation state handling

The changes improve state handling during client-side navigation by: -
Using correct function updates with setAppState - Preserving abort
controller on updates - Adding proper cache ID comparison - Moving
router instance to constants

packages/bun-framework-react/client.tsx

@@ -1,10 +1,8 @@
 import { onServerSideReload } from "bun:app/client";
 import { hydrateRoot } from "react-dom/client";
 import { initialRscPayloadThen } from "./client/app.ts";
+import { router } from "./client/constants.ts";
 import { Root } from "./client/root.tsx";
-import { Router } from "./client/router.ts";
-
-const router = new Router();
 
 hydrateRoot(document, <Root />, {
   onUncaughtError(e) {
@@ -50,13 +48,8 @@ const firstPageId = Date.now();
   }
 }
 
-// Client side navigation is implemented by updating the app's `useState` with a
-// new RSC payload promise. Callers of `navigate` are expected to manage history
-// state. A navigation id is used
-
 window.addEventListener("popstate", async event => {
   const state = typeof event.state === "number" ? event.state : undefined;
-
   await router.navigate(location.href, state);
 });
 

packages/bun-framework-react/client/router.ts

@@ -109,7 +109,7 @@ export class Router {
 
     // Save this promise so that pressing the back button in the browser navigates
     // to the same instance of the old page, instead of re-fetching it.
-    if (cacheId) {
+    if (cacheId !== undefined) {
       this.cachedPages.set(cacheId, {
         css: [...this.css.getList()],
         element: p,
@@ -124,21 +124,22 @@ export class Router {
     }
 
     // Tell react about the new page promise
-    if (document.startViewTransition as unknown) {
+    if (document.startViewTransition) {
       document.startViewTransition(() => {
         flushSync(() => {
-          if (thisNavigationId === this.lastNavigationId)
-            setAppState({
+          if (thisNavigationId === this.lastNavigationId) {
+            setAppState(old => ({
               rsc: p,
-              abortOnRender: olderController ?? undefined,
-            });
+              abortOnRender: olderController ?? old.abortOnRender,
+            }));
+          }
         });
       });
     } else {
-      setAppState({
+      setAppState(old => ({
         rsc: p,
-        abortOnRender: olderController ?? undefined,
-      });
+        abortOnRender: olderController ?? old.abortOnRender,
+      }));
     }
   }
 }