diff --git a/public/sw.js b/public/sw.js
index e56c47a381..b4a3f57220 100644
--- a/public/sw.js
+++ b/public/sw.js
@@ -72,9 +72,14 @@ self.addEventListener("fetch", (event) => {
     (async () => {
       if (isNavigateRequest) {
         try {
-          return await fetch(event.request);
+          const networkResponse = await fetch(event.request);
+          if (networkResponse && networkResponse.status === 200) {
+            const responseClone = networkResponse.clone();
+            void caches.open(CACHE_NAME).then((cache) => cache.put(event.request, responseClone));
+          }
+          return networkResponse;
         } catch {
-          return (await caches.match("/offline")) || Response.error();
+          return (await navigationFallback(event.request)) || Response.error();
         }
       }
 
@@ -110,6 +115,10 @@ self.addEventListener("fetch", (event) => {
   );
 });
 
+async function navigationFallback(request) {
+  return (await caches.match(request)) || (await caches.match("/")) || (await caches.match("/offline"));
+}
+
 // ── Push Notifications ───────────────────────────────────────────────────────
 
 self.addEventListener("push", (event) => {
diff --git a/tests/unit/service-worker-navigation-fallback.test.ts b/tests/unit/service-worker-navigation-fallback.test.ts
new file mode 100644
index 0000000000..9b5a4fea01
--- /dev/null
+++ b/tests/unit/service-worker-navigation-fallback.test.ts
@@ -0,0 +1,123 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { readFileSync } from "node:fs";
+import vm from "node:vm";
+
+type FetchEvent = {
+  request: RequestLike;
+  respondWith: (response: Promise<Response>) => void;
+};
+
+type ServiceWorkerEvent = FetchEvent | Record<string, unknown>;
+
+type RequestLike = {
+  url: string;
+  method: string;
+  mode: string;
+  destination: string;
+};
+
+function createServiceWorkerHarness() {
+  const cacheEntries = new Map<string, Response>();
+  const listeners = new Map<string, (event: ServiceWorkerEvent) => void>();
+  let fetchImpl: (request: RequestLike) => Promise<Response> = async () => {
+    throw new Error("network unavailable");
+  };
+
+  const cache = {
+    addAll: async (urls: string[]) => {
+      for (const url of urls) {
+        cacheEntries.set(url, new Response(`cached ${url}`, { status: 200 }));
+      }
+    },
+    delete: async (request: Request) => cacheEntries.delete(request.url),
+    keys: async () => [...cacheEntries.keys()].map((url) => new Request(url)),
+    put: async (request: Request, response: Response) => {
+      cacheEntries.set(request.url, response);
+    },
+  };
+
+  const caches = {
+    delete: async () => true,
+    keys: async () => ["omniroute-pwa-v2"],
+    match: async (request: Request | string) =>
+      cacheEntries.get(typeof request === "string" ? request : request.url),
+    open: async () => cache,
+  };
+
+  const context = vm.createContext({
+    URL,
+    Request,
+    Response,
+    caches,
+    fetch: (request: RequestLike) => fetchImpl(request),
+    self: {
+      clients: { claim: async () => undefined },
+      location: { href: "https://app.example/sw.js", origin: "https://app.example" },
+      registration: { showNotification: async () => undefined },
+      skipWaiting: async () => undefined,
+      addEventListener: (type: string, listener: (event: ServiceWorkerEvent) => void) => {
+        listeners.set(type, listener);
+      },
+    },
+  });
+
+  vm.runInContext(readFileSync("public/sw.js", "utf8"), context);
+
+  return {
+    cacheEntries,
+    dispatchFetch: async (request: RequestLike) => {
+      const listener = listeners.get("fetch");
+      assert.ok(listener, "fetch listener must be registered");
+      let responsePromise: Promise<Response> | undefined;
+      const event: FetchEvent = {
+        request,
+        respondWith: (response) => {
+          responsePromise = response;
+        },
+      };
+      listener(event);
+      assert.ok(responsePromise, "navigate request must call respondWith");
+      return responsePromise;
+    },
+    setFetch: (nextFetch: (request: Request) => Promise<Response>) => {
+      fetchImpl = nextFetch;
+    },
+  };
+}
+
+test("#5165: service worker returns cached navigation before offline page", async () => {
+  const harness = createServiceWorkerHarness();
+  const request = {
+    url: "https://app.example/dashboard",
+    method: "GET",
+    mode: "navigate",
+    destination: "document",
+  };
+
+  harness.cacheEntries.set(request.url, new Response("cached dashboard", { status: 200 }));
+  harness.cacheEntries.set("/offline", new Response("offline page", { status: 200 }));
+
+  const response = await harness.dispatchFetch(request);
+
+  assert.equal(await response.text(), "cached dashboard");
+});
+
+test("#5165: successful navigations are cached for later transient failures", async () => {
+  const harness = createServiceWorkerHarness();
+  const request = {
+    url: "https://app.example/dashboard",
+    method: "GET",
+    mode: "navigate",
+    destination: "document",
+  };
+
+  harness.setFetch(async () => new Response("fresh dashboard", { status: 200 }));
+  assert.equal(await (await harness.dispatchFetch(request)).text(), "fresh dashboard");
+
+  harness.setFetch(async () => {
+    throw new Error("transient network failure");
+  });
+
+  assert.equal(await (await harness.dispatchFetch(request)).text(), "fresh dashboard");
+});