v3: various improvements and fixes to real-time updates

src/pages/Index.tsx

@@ -70,6 +70,7 @@ export function Index({ categorySlug }: { categorySlug?: string }) {
     deletedDumpIds,
     castVote,
     removeVote,
+    connectionEpoch,
   } = useWS();
 
   // Resolve the scope to a real category (for its id, name, and 404 handling).
@@ -177,6 +178,36 @@ export function Index({ categorySlug }: { categorySlug?: string }) {
   );
   useDumpListSync(setDumpsItems);
 
+  // ── Reconnect backfill ──
+
+  // New dumps that arrived while the socket was down weren't delivered. On
+  // reconnect, re-fetch page 1 and merge anything not already present on top,
+  // preserving loaded pages and scroll position.
+  const backfillEpochRef = useRef(connectionEpoch);
+  useEffect(() => {
+    if (connectionEpoch === backfillEpochRef.current) return;
+    backfillEpochRef.current = connectionEpoch;
+    const controller = new AbortController();
+    fetch(feedUrl(1), {
+      signal: controller.signal,
+      headers: token ? { Authorization: `Bearer ${token}` } : {},
+    })
+      .then((r) => r.json())
+      .then((body) => {
+        const { items } = body.data as PaginatedData<RawDump>;
+        const fresh = items.map(deserializeDump);
+        setDumpsState((s) => {
+          if (s.status !== "loaded") return s;
+          const known = new Set(s.dumps.map((d) => d.id));
+          const newOnes = fresh.filter((d) => !known.has(d.id));
+          if (newOnes.length === 0) return s;
+          return { ...s, dumps: [...newOnes, ...s.dumps] };
+        });
+      })
+      .catch(() => {});
+    return () => controller.abort();
+  }, [connectionEpoch, feedUrl, token]);
+
   // ── Load more ──
 
   const loadMore = useCallback(() => {