fix(library): keep the shell the deploy replaced out of a failed navigation, and say when another tab holds the library open

docker/frontend/public/sw.js

@@ -37,9 +37,15 @@ const cachedFetch = (request) =>
     return response;
   });
 
-// Offline, the studio the app is for; the landing page for a build without /app.
-const offlineShell = async () =>
-  (await caches.match('/app')) ?? (await caches.match('/')) ?? Response.error();
+// Offline, the route that was asked for if this browser has ever been on it, then
+// the studio the app is for, then the landing page for a build without /app. The
+// route first because that is the copy a visit keeps up to date: the other two may
+// be shells of a build the deploy has replaced.
+const offlineShell = async (request) =>
+  (await caches.match(new Request(new URL(request.url).pathname))) ??
+  (await caches.match('/app')) ??
+  (await caches.match('/')) ??
+  Response.error();
 
 self.addEventListener('install', (event) => {
   // Best effort, one route at a time: a deploy caught mid-flight must not leave the
@@ -73,12 +79,28 @@ self.addEventListener('fetch', (event) => {
   // `no-store` because a plain fetch is not the network: it may be answered by the
   // browser's own cache, which is the other place a stale shell hides.
   if (request.mode === 'navigate') {
-    event.respondWith(fetch(request, { cache: 'no-store' }).catch(offlineShell));
+    event.respondWith(
+      fetch(request, { cache: 'no-store' })
+        .then((response) => {
+          // The shell kept for offline is the one the server handed over last, not
+          // the one this worker installed with: a navigation that fails — a deploy
+          // recreating the container is the ordinary way one does — otherwise
+          // answers with the build of the first visit, `activate` never runs to
+          // replace it while this worker's bytes stay the same, and the browser
+          // runs an old build until a reload happens to land after the server is
+          // back. Stored under the route the navigation asked for, which is one of
+          // the three SHELL keys — a navigation request is not a key a cache takes.
+          if (response.ok) {
+            const copy = response.clone();
+            const key = new Request(new URL(request.url).pathname);
+            caches.open(VERSION).then((cache) => cache.put(key, copy)).catch(() => {});
+          }
+          return response;
+        })
+        .catch(() => offlineShell(request))
+    );
     return;
   }
-
-  // ponytail: a navigation is never written back, the precached entry is what
-  // answers offline. Add a put() here once a route must survive on its own.
   if (STATIC.test(url.pathname)) {
     event.respondWith(caches.match(request).then((hit) => hit ?? cachedFetch(request)));
     return;