fix: duplicate messages on scroll-to-top in chat

Three root causes fixed:
- Room Flow observer overwrote deduplicated list during loadMoreHistory()
  → Added isLoadingHistory flag to suppress Flow updates while loading
- beforeId used local IDs (user_/asst_ prefix) that server couldn't resolve,
  causing server to return ALL messages → Find oldest server-UUID message as cursor
- Scroll-to-top LaunchedEffect could double-fire → Added 800ms throttle

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

android/app/src/main/java/com/tiangong/aiagent/ui/chat/ChatScreen.kt

@@ -85,7 +85,7 @@ fun ChatScreen(
         }
     }
 
-    // Scroll-to-top detection for loading more history
+    // Scroll-to-top detection for loading more history (throttled to prevent double-fire)
     val isAtTop by remember {
         derivedStateOf {
             listState.firstVisibleItemIndex == 0 &&
@@ -93,8 +93,14 @@ fun ChatScreen(
         }
     }
 
+    var lastLoadMoreTime by remember { mutableStateOf(0L) }
+
     LaunchedEffect(isAtTop) {
-        if (isAtTop && uiState.hasMoreMessages && !uiState.isLoadingMore && initialScrollDone) {
+        val now = System.currentTimeMillis()
+        if (isAtTop && uiState.hasMoreMessages && !uiState.isLoadingMore && initialScrollDone
+            && (now - lastLoadMoreTime) > 800
+        ) {
+            lastLoadMoreTime = now
             viewModel.loadMoreHistory()
         }
     }

android/app/src/main/java/com/tiangong/aiagent/ui/chat/ChatViewModel.kt

@@ -124,6 +124,7 @@ class ChatViewModel @Inject constructor(
 
     private var sseJob: Job? = null
     private var historyFlowJob: Job? = null
+    private var isLoadingHistory = false
 
     companion object {
         private const val KEY_SESSION_STATE = "chat_session_state"
@@ -181,9 +182,9 @@ class ChatViewModel @Inject constructor(
         val currentAgentId = _uiState.value.currentAgent?.id
         historyFlowJob = viewModelScope.launch {
             chatRepository.getMessages(sessionId).collect { messages ->
-                if (messages.isNotEmpty() && _uiState.value.currentAgent?.id == currentAgentId) {
+                if (messages.isNotEmpty() && _uiState.value.currentAgent?.id == currentAgentId && !isLoadingHistory) {
                     _uiState.value = _uiState.value.copy(
-                        messages = messages.map { it.toUiMessage() }.distinctBy { it.id }.distinctBy { it.id },
+                        messages = messages.map { it.toUiMessage() }.distinctBy { it.id },
                         sessionId = sessionId
                     )
                 }
@@ -243,7 +244,7 @@ class ChatViewModel @Inject constructor(
             val latestConversation = conversations.firstOrNull()
             if (latestConversation != null) {
                 chatRepository.getMessages(latestConversation.sessionId).collect { messages ->
-                    if (messages.isNotEmpty() && _uiState.value.currentAgent?.id == agentId) {
+                    if (messages.isNotEmpty() && _uiState.value.currentAgent?.id == agentId && !isLoadingHistory) {
                         _uiState.value = _uiState.value.copy(
                             messages = messages.map { it.toUiMessage() }.distinctBy { it.id },
                             sessionId = latestConversation.sessionId
@@ -312,7 +313,7 @@ class ChatViewModel @Inject constructor(
                 historyFlowJob?.cancel()
                 historyFlowJob = viewModelScope.launch {
                     chatRepository.getMessages(sessionId).collect { messages ->
-                        if (messages.isNotEmpty()) {
+                        if (messages.isNotEmpty() && !isLoadingHistory) {
                             _uiState.value = _uiState.value.copy(
                                 messages = messages.map { it.toUiMessage() }.distinctBy { it.id }
                             )
@@ -350,10 +351,15 @@ class ChatViewModel @Inject constructor(
         val oldestMsg = state.messages.firstOrNull() ?: return
 
         viewModelScope.launch {
+            isLoadingHistory = true
             _uiState.value = _uiState.value.copy(isLoadingMore = true)
 
-            // Capture current scroll anchor (first visible item id + offset)
-            val anchorId = oldestMsg.id
+            // Use a server-resolvable ID as cursor: local IDs (user_/asst_/tool_/system_ prefix)
+            // are not in the server DB, so the server would ignore the cursor and return ALL messages.
+            // Find the oldest message whose ID looks like a server UUID.
+            val serverIdPattern = Regex("^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$", RegexOption.IGNORE_CASE)
+            val anchorId = state.messages.firstOrNull { serverIdPattern.matches(it.id) }?.id
+                ?: oldestMsg.id
 
             val result = chatRepository.fetchOlderMessages(
                 agentId = agentId,
@@ -377,6 +383,7 @@ class ChatViewModel @Inject constructor(
                     hasMoreMessages = hasMore,
                     isLoadingMore = false
                 )
+                isLoadingHistory = false
             } else {
                 // API failed — try Room fallback
                 val oldestTimestamp = oldestMsg.createdAt
@@ -398,11 +405,13 @@ class ChatViewModel @Inject constructor(
                         hasMoreMessages = roomMsgs.size >= 50,
                         isLoadingMore = false
                     )
+                    isLoadingHistory = false
                 } else {
                     _uiState.value = _uiState.value.copy(
                         hasMoreMessages = false,
                         isLoadingMore = false
                     )
+                    isLoadingHistory = false
                 }
             }
         }