perf(room): prioritize IndexedDB cache for instant loads

- Remove clearRoomMessages on room switch: IDB cache now persists across
  visits, enabling instant re-entry without API round-trip
- Increase initial API limit from 50 → 200: more messages cached upfront
- Add loadOlderMessagesFromIdb: uses 'by_room_seq' compound index to
  serve scroll-back history from IDB without API call
- loadMore now tries IDB first before falling back to API

src/contexts/room-context.tsx

@@ -32,8 +32,8 @@ import {
   saveMessage,
   saveMessages,
   loadMessages as loadMessagesFromIdb,
+  loadOlderMessagesFromIdb,
   deleteMessage as deleteMessageFromIdb,
-  clearRoomMessages,
 } from '@/lib/storage/indexed-db';
 
 export type { RoomWsStatus, RoomWsClient } from '@/lib/room-ws-client';
@@ -231,10 +231,8 @@ export function RoomProvider({
       setMessages([]);
       setIsHistoryLoaded(false);
       setNextCursor(null);
-      // Clear old room's IDB cache asynchronously (fire and forget)
-      if (oldRoomId) {
-        clearRoomMessages(oldRoomId).catch(() => {});
-      }
+      // NOTE: intentionally NOT clearing IndexedDB — keeping it enables instant
+      // load when the user returns to this room without waiting for API.
     }
   }, [activeRoomId]);
 
@@ -274,31 +272,56 @@ export function RoomProvider({
 
       setIsLoadingMore(true);
       try {
-        // Initial load: check IndexedDB first for fast render
-        if (cursor === null || cursor === undefined) {
+        const isInitial = cursor === null || cursor === undefined;
+        const limit = isInitial ? 200 : 50;
+
+        // --- Initial load: try IndexedDB first for instant render ---
+        if (isInitial) {
           const cached = await loadMessagesFromIdb(activeRoomId);
           if (cached.length > 0) {
             setMessages(cached);
             setIsTransitioningRoom(false);
-            // Derive cursor from IDB data (oldest message's seq = cursor)
             const minSeq = cached[0].seq;
             setNextCursor(minSeq > 0 ? minSeq - 1 : null);
-            // If IDB has data, skip API call — WS will push live updates
-            // Still set isLoadingMore to false and return
             setIsLoadingMore(false);
+            // No API call needed — WS will push any new messages that arrived while away
             return;
           }
         }
 
-        // Call API (IDB was empty on initial load, or user is loading older history)
+        // --- Load older history: try IDB first, then fall back to API ---
+        if (!isInitial && cursor != null) {
+          const idbMessages = await loadOlderMessagesFromIdb(activeRoomId, cursor, limit);
+          if (idbMessages.length > 0) {
+            setMessages((prev) => {
+              if (abortController.signal.aborted) return prev;
+              const existingIds = new Set(prev.map((m) => m.id));
+              const filtered = idbMessages.filter((m) => !existingIds.has(m.id));
+              let merged = [...filtered, ...prev];
+              merged.sort((a, b) => a.seq - b.seq);
+              if (merged.length > MAX_MESSAGES_IN_MEMORY) {
+                merged = merged.slice(-MAX_MESSAGES_IN_MEMORY);
+              }
+              return merged;
+            });
+            const oldest = idbMessages[0];
+            setNextCursor(oldest.seq > 0 ? oldest.seq - 1 : null);
+            if (idbMessages.length < limit) {
+              setIsHistoryLoaded(true);
+            }
+            setIsLoadingMore(false);
+            return;
+          }
+          // IDB empty for this range — fall through to API
+        }
+
+        // --- API fetch ---
         const resp = await client.messageList(activeRoomId, {
           beforeSeq: cursor ?? undefined,
-          limit: 50,
+          limit,
         });
 
-        if (abortController.signal.aborted) {
-          return;
-        }
+        if (abortController.signal.aborted) return;
 
         const newMessages = resp.messages.map((m) => ({
           ...m,
@@ -308,17 +331,11 @@ export function RoomProvider({
         }));
 
         setMessages((prev) => {
-          // Double-check room hasn't changed
-          if (abortController.signal.aborted) {
-            return prev;
-          }
-          // If initial load (cursor=null), replace instead of merge (room switching)
-          if (cursor === null || cursor === undefined) {
-            // Clear transitioning state
+          if (abortController.signal.aborted) return prev;
+          if (isInitial) {
             setIsTransitioningRoom(false);
             return newMessages;
           }
-          // loadMore: prepend older messages before existing
           const existingIds = new Set(prev.map((m) => m.id));
           const filtered = newMessages.filter((m) => !existingIds.has(m.id));
           let merged = [...filtered, ...prev];
@@ -329,18 +346,16 @@ export function RoomProvider({
           return merged;
         });
 
-        // Persist new messages to IndexedDB
         if (newMessages.length > 0) {
           saveMessages(activeRoomId, newMessages).catch(() => {});
         }
 
-        if (resp.messages.length < 50) {
+        if (resp.messages.length < limit) {
           setIsHistoryLoaded(true);
         }
-        // messages are in ascending order (oldest first), so [length-1] is newest
         setNextCursor(resp.messages.length > 0 ? resp.messages[resp.messages.length - 1].seq : null);
 
-        // Fetch reactions for all loaded messages (backend may not support this yet)
+        // Fetch reactions for all loaded messages
         const msgIds = newMessages.map((m) => m.id);
         if (msgIds.length > 0) {
           try {
@@ -363,10 +378,7 @@ export function RoomProvider({
           }
         }
       } catch (error) {
-        // Ignore abort errors
-        if (abortController.signal.aborted) {
-          return;
-        }
+        if (abortController.signal.aborted) return;
         handleRoomError('Load messages', error);
       } finally {
         setIsLoadingMore(false);

src/lib/storage/indexed-db.ts

@@ -183,6 +183,44 @@ export async function clearRoomMessages(roomId: string): Promise<void> {
   }
 }
 
+/** Load older messages from IDB (seq < beforeSeq), sorted ascending, up to `limit` */
+export async function loadOlderMessagesFromIdb(
+  roomId: string,
+  beforeSeq: number,
+  limit = 50,
+): Promise<MessageWithMeta[]> {
+  try {
+    const db = await openDB();
+    const tx = db.transaction(STORE_MESSAGES, 'readonly');
+    const index = tx.objectStore(STORE_MESSAGES).index('by_room_seq');
+
+    // Compound key range: roomId + any seq less than beforeSeq
+    const range = IDBKeyRange.bound([roomId, 0], [roomId, beforeSeq - 1]);
+    const request = index.openCursor(range, 'prev'); // 'prev' = descending seq (newest first)
+    // We want oldest before `beforeSeq`, so after getting `limit` items in 'prev' order,
+    // reverse to ascending seq.
+    const collected: StoredMessage[] = [];
+
+    return new Promise((resolve, reject) => {
+      request.onsuccess = () => {
+        const cursor = request.result;
+        if (cursor && collected.length < limit) {
+          collected.push(cursor.value);
+          cursor.continue();
+        } else {
+          // Reverse back to ascending order (oldest first)
+          const msgs = collected.reverse().map(storedToMsg);
+          resolve(msgs);
+        }
+      };
+      request.onerror = () => reject(request.error);
+    });
+  } catch (err) {
+    console.warn('[IDB] loadOlderMessagesFromIdb failed:', err);
+    return [];
+  }
+}
+
 /** Get the highest seq number for a room (for dedup) */
 export async function getMaxSeq(roomId: string): Promise<number> {
   try {