fix(chat): render cached history before network sync

src/stores/chat/chat-history-sync.ts

@@ -17,7 +17,17 @@ export type ReadAndSyncHistoryOutput = {
   networkCount: number;
 };
 
-function createGreetingMessage(): UiMessage {
+export type LocalHistorySnapshotOutput = {
+  /** Local cached messages. Empty local history includes a UI greeting. */
+  messages: UiMessage[];
+  hasMore: boolean;
+  newOffset: number;
+  localCount: number;
+};
+
+export type NetworkHistorySyncOutput = ReadAndSyncHistoryOutput;
+
+export function createGreetingMessage(): UiMessage {
   return {
     content:
       "You're here! Facebook is so restrictive, " +
@@ -28,13 +38,7 @@ function createGreetingMessage(): UiMessage {
   };
 }
 
-/**
- * History sync flow:
- * 1. Read local history for fallback.
- * 2. Read network history as the authoritative source.
- * 3. Overwrite local history with network data.
- */
-export async function readAndSyncHistory(): Promise<ReadAndSyncHistoryOutput> {
+export async function readLocalHistorySnapshot(): Promise<LocalHistorySnapshotOutput> {
   const chatRepo = getChatRepository();
   const greetingMessage = createGreetingMessage();
 
@@ -47,27 +51,32 @@ export async function readAndSyncHistory(): Promise<ReadAndSyncHistoryOutput> {
     count: localMessages.length,
   });
 
+  const snapshotMessages =
+    localMessages.length === 0 ? [greetingMessage] : localMessages;
+  if (snapshotMessages[0] === greetingMessage) {
+    log.debug("[chat-machine] loadHistory EMPTY -> prepend greeting (local)");
+  }
+
+  return {
+    messages: snapshotMessages,
+    hasMore: localMessages.length >= PAGE_SIZE,
+    newOffset: snapshotMessages.length,
+    localCount: localMessages.length,
+  };
+}
+
+export async function syncNetworkHistory(
+  localCount: number,
+): Promise<NetworkHistorySyncOutput | null> {
+  const chatRepo = getChatRepository();
+  const greetingMessage = createGreetingMessage();
+
   const networkResult = await chatRepo.getHistory(PAGE_SIZE, 0);
   if (Result.isErr(networkResult)) {
     log.error("[chat-machine] loadHistory NETWORK FAILED", {
       error: networkResult.error,
     });
-
-    const fallbackMessages =
-      localMessages.length === 0 ? [greetingMessage] : localMessages;
-    if (fallbackMessages[0] === greetingMessage) {
-      log.debug(
-        "[chat-machine] loadHistory EMPTY -> prepend greeting (network-failed path)",
-      );
-    }
-    return {
-      messages: fallbackMessages,
-      hasMore: false,
-      newOffset: fallbackMessages.length,
-      localOverwritten: false,
-      localCount: localMessages.length,
-      networkCount: 0,
-    };
+    return null;
   }
 
   const networkUi = localMessagesToUi(networkResult.data.messages);
@@ -95,7 +104,28 @@ export async function readAndSyncHistory(): Promise<ReadAndSyncHistoryOutput> {
     hasMore: finalMessages.length >= PAGE_SIZE,
     newOffset: finalMessages.length,
     localOverwritten,
-    localCount: localMessages.length,
+    localCount,
     networkCount: networkUi.length,
   };
 }
+
+/**
+ * History sync flow:
+ * 1. Read local history for fallback.
+ * 2. Read network history as the authoritative source.
+ * 3. Overwrite local history with network data.
+ */
+export async function readAndSyncHistory(): Promise<ReadAndSyncHistoryOutput> {
+  const localSnapshot = await readLocalHistorySnapshot();
+  const networkSnapshot = await syncNetworkHistory(localSnapshot.localCount);
+  if (networkSnapshot) return networkSnapshot;
+
+  return {
+    messages: localSnapshot.messages,
+    hasMore: localSnapshot.hasMore,
+    newOffset: localSnapshot.newOffset,
+    localOverwritten: false,
+    localCount: localSnapshot.localCount,
+    networkCount: 0,
+  };
+}