feat(mobile): lock chat input while bot is responding + 20s timeout (#1538)

* feat(mobile): lock chat input while bot is responding + 20s timeout

- Add _isWaitingForResponse flag to ChatProvider; set in _startPolling,
  cleared in _stopPolling so it covers the full polling lifecycle not
  just the initial HTTP POST
- Add _pollingStartTime + 20s timeout in _pollForUpdates; if the bot
  never responds the flag resets, errorMessage is surfaced, and input
  unlocks automatically
- Gate send button and keyboard shortcut on isSendingMessage ||
  isWaitingForResponse so users cannot queue up multiple messages
  while a response is in flight

(adding an interrupt like with other chat bots would require a larger rewrite of the backend structure)

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

* fix(mobile): make polling timeout measure inactivity not total duration

Reset _pollingStartTime whenever assistant content grows so the 20s
timeout only fires if no new content has arrived in that window.
Prevents cutting off a slow-but-streaming response mid-generation.

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

* fix(mobile): lock input for full polling duration, not just until first chunk

- Add isPolling getter to ChatProvider (true while _pollingTimer is active)
- Gate send button and intent on isPolling in addition to isWaitingForResponse
  so users cannot submit overlapping prompts while a response is still streaming
- Also auto-scroll while polling is active

* Fix chat polling timeout race and send re-entry guard

Polling timeout was evaluated before the network attempt, allowing it
to fire just as a response became ready. Timeout check now runs after
each poll attempt and only when no progress was made; network errors
fall through to the same check instead of silently swallowing the tick.

Added _isSendInFlight boolean to prevent rapid taps from re-entering
_sendMessage() during the async token fetch window before provider
flags are set. Guard is set synchronously at the top of the method and
cleared in a finally block.

* fix(mobile): prevent overlapping polls and empty-placeholder stop

Add _isPollingRequestInFlight guard so Timer.periodic ticks are
skipped if a getChat request is still in flight, preventing stale
results from resetting state out of order.

Fix empty assistant placeholder incorrectly triggering _stopPolling:
stable is only declared when a previously observed length exists and
hasn't grown. An initial empty message keeps polling until content
arrives or the timeout fires.

* fix(mobile): reset _lastAssistantContentLength in _stopPolling

Prevents stale content-length state from a prior polling session
bleeding into the next one.

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

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

mobile/lib/providers/chat_provider.dart

@@ -14,6 +14,10 @@ class ChatProvider with ChangeNotifier {
   bool _isWaitingForResponse = false;
   String? _errorMessage;
   Timer? _pollingTimer;
+  DateTime? _pollingStartTime;
+  bool _isPollingRequestInFlight = false;
+
+  static const _pollingTimeout = Duration(seconds: 20);
 
   /// Content length of the last assistant message from the previous poll.
   /// Used to detect when the LLM has finished writing (no growth between polls).
@@ -24,6 +28,7 @@ class ChatProvider with ChangeNotifier {
   bool get isLoading => _isLoading;
   bool get isSendingMessage => _isSendingMessage;
   bool get isWaitingForResponse => _isWaitingForResponse;
+  bool get isPolling => _pollingTimer != null;
   String? get errorMessage => _errorMessage;
 
   /// Fetch list of chats
@@ -262,10 +267,17 @@ class ChatProvider with ChangeNotifier {
     _pollingTimer?.cancel();
     _lastAssistantContentLength = null;
     _isWaitingForResponse = true;
+    _pollingStartTime = DateTime.now();
     notifyListeners();
 
     _pollingTimer = Timer.periodic(const Duration(seconds: 2), (timer) async {
-      await _pollForUpdates(accessToken, chatId);
+      if (_isPollingRequestInFlight) return;
+      _isPollingRequestInFlight = true;
+      try {
+        await _pollForUpdates(accessToken, chatId);
+      } finally {
+        _isPollingRequestInFlight = false;
+      }
     });
   }
 
@@ -273,7 +285,10 @@ class ChatProvider with ChangeNotifier {
   void _stopPolling() {
     _pollingTimer?.cancel();
     _pollingTimer = null;
+    _pollingStartTime = null;
+    _isPollingRequestInFlight = false;
     _isWaitingForResponse = false;
+    _lastAssistantContentLength = null;
   }
 
   /// Poll for updates
@@ -333,19 +348,39 @@ class ChatProvider with ChangeNotifier {
         final lastMessage = updatedChat.messages.lastOrNull;
         if (lastMessage != null && lastMessage.isAssistant) {
           final newLen = lastMessage.content.length;
-          if (newLen > (_lastAssistantContentLength ?? 0)) {
+          final previousLen = _lastAssistantContentLength;
+
+          if (newLen > (previousLen ?? -1)) {
             _lastAssistantContentLength = newLen;
-          } else {
-            // Content stable: no growth since last poll — done.
+            if (newLen > 0) {
+              // Content is growing — reset the inactivity clock.
+              _pollingStartTime = DateTime.now();
+              return; // progress made, don't evaluate timeout this tick
+            }
+            // newLen == 0: empty placeholder, keep polling
+          } else if (newLen > 0) {
+            // Content stable and non-empty: no growth since last poll — done.
             _stopPolling();
             _lastAssistantContentLength = null;
             notifyListeners();
+            return;
           }
+          // newLen == 0 with previousLen already 0: still empty, keep polling
         }
       }
     } catch (e) {
+      // Network error — allow polling to continue; timeout check below will
+      // stop it if the deadline has passed.
       debugPrint('Polling error: ${e.toString()}');
     }
+
+    // Evaluate timeout only after the attempt, and only when no progress was made.
+    if (_pollingStartTime != null &&
+        DateTime.now().difference(_pollingStartTime!) >= _pollingTimeout) {
+      _stopPolling();
+      _errorMessage = 'The assistant took too long to respond. Please try again.';
+      notifyListeners();
+    }
   }
 
   /// Clear current chat

mobile/lib/screens/chat_conversation_screen.dart

@@ -34,6 +34,7 @@ class _ChatConversationScreenState extends State<ChatConversationScreen> {
 
   ChatProvider? _chatProvider;
   bool _listenerAdded = false;
+  bool _isSendInFlight = false;
 
   @override
   void initState() {
@@ -68,7 +69,7 @@ class _ChatConversationScreenState extends State<ChatConversationScreen> {
   void _onChatChanged() {
     if (!mounted) return;
     final chatProvider = Provider.of<ChatProvider>(context, listen: false);
-    if (chatProvider.isWaitingForResponse || chatProvider.isSendingMessage) {
+    if (chatProvider.isWaitingForResponse || chatProvider.isSendingMessage || chatProvider.isPolling) {
       WidgetsBinding.instance.addPostFrameCallback((_) {
         if (mounted) _scrollToBottom();
       });
@@ -120,9 +121,12 @@ class _ChatConversationScreenState extends State<ChatConversationScreen> {
   }
 
   Future<void> _sendMessage() async {
+    if (_isSendInFlight) return;
     final content = _messageController.text.trim();
     if (content.isEmpty) return;
+    setState(() => _isSendInFlight = true);
 
+    try {
     final authProvider = Provider.of<AuthProvider>(context, listen: false);
     final chatProvider = Provider.of<ChatProvider>(context, listen: false);
 
@@ -182,6 +186,9 @@ class _ChatConversationScreenState extends State<ChatConversationScreen> {
         );
       }
     });
+    } finally {
+      if (mounted) setState(() => _isSendInFlight = false);
+    }
   }
 
   Future<void> _editTitle() async {
@@ -359,7 +366,7 @@ class _ChatConversationScreenState extends State<ChatConversationScreen> {
                     actions: <Type, Action<Intent>>{
                       _SendMessageIntent: CallbackAction<_SendMessageIntent>(
                         onInvoke: (_) {
-                          if (!chatProvider.isSendingMessage) _sendMessage();
+                          if (!_isSendInFlight && !chatProvider.isSendingMessage && !chatProvider.isWaitingForResponse && !chatProvider.isPolling) _sendMessage();
                           return null;
                         },
                       ),
@@ -387,7 +394,7 @@ class _ChatConversationScreenState extends State<ChatConversationScreen> {
                         const SizedBox(width: 8),
                         IconButton(
                           icon: const Icon(Icons.send),
-                          onPressed: chatProvider.isSendingMessage
+                          onPressed: (_isSendInFlight || chatProvider.isSendingMessage || chatProvider.isWaitingForResponse || chatProvider.isPolling)
                               ? null
                               : _sendMessage,
                           color: colorScheme.primary,