fix(terminal): reconnect, error handling

1. Terminal shows "^[[1;1R^[[2;2R^[[>0;0;0c"
2. NaN

```
[ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: Converting object to an encodable object failed: NaN
...
```

Signed-off-by: fufesou <linlong1266@gmail.com>

flutter/lib/desktop/pages/terminal_page.dart

@@ -43,6 +43,9 @@ class TerminalPage extends StatefulWidget {
 
 class _TerminalPageState extends State<TerminalPage>
     with AutomaticKeepAliveClientMixin {
+  static const EdgeInsets _defaultTerminalPadding =
+      EdgeInsets.symmetric(horizontal: 5.0, vertical: 2.0);
+
   late FFI _ffi;
   late TerminalModel _terminalModel;
   double? _cellHeight;
@@ -155,11 +158,22 @@ class _TerminalPageState extends State<TerminalPage>
   // extra space left after dividing the available height by the height of a single
   // terminal row (`_cellHeight`) and distributing it evenly as top and bottom padding.
   EdgeInsets _calculatePadding(double heightPx) {
-    if (_cellHeight == null) {
-      return const EdgeInsets.symmetric(horizontal: 5.0, vertical: 2.0);
+    final cellHeight = _cellHeight;
+    if (!heightPx.isFinite ||
+        heightPx <= 0 ||
+        cellHeight == null ||
+        !cellHeight.isFinite ||
+        cellHeight <= 0) {
+      return _defaultTerminalPadding;
+    }
+    final rows = (heightPx / cellHeight).floor();
+    if (rows <= 0) {
+      return _defaultTerminalPadding;
+    }
+    final extraSpace = heightPx - rows * cellHeight;
+    if (!extraSpace.isFinite || extraSpace < 0) {
+      return _defaultTerminalPadding;
     }
-    final rows = (heightPx / _cellHeight!).floor();
-    final extraSpace = heightPx - rows * _cellHeight!;
     final topBottom = extraSpace / 2.0;
     return EdgeInsets.symmetric(horizontal: 5.0, vertical: topBottom);
   }

flutter/lib/models/terminal_model.dart

@@ -27,10 +27,13 @@ class TerminalModel with ChangeNotifier {
   // Buffer for output data received before terminal view has valid dimensions.
   // This prevents NaN errors when writing to terminal before layout is complete.
   final _pendingOutputChunks = <String>[];
+  final _pendingOutputSuppressFlags = <bool>[];
   int _pendingOutputSize = 0;
   static const int _kMaxOutputBufferChars = 8 * 1024;
   // View ready state: true when terminal has valid dimensions, safe to write
   bool _terminalViewReady = false;
+  bool _suppressTerminalOutput = false;
+  bool _suppressNextTerminalDataOutput = false;
 
   void Function(int w, int h, int pw, int ph)? onResizeExternal;
 
@@ -71,7 +74,10 @@ class TerminalModel with ChangeNotifier {
     terminalController = TerminalController();
 
     // Setup terminal callbacks
-    terminal.onOutput = _handleInput;
+    terminal.onOutput = (data) {
+      if (_suppressTerminalOutput) return;
+      _handleInput(data);
+    };
 
     terminal.onResize = (w, h, pw, ph) async {
       // Validate all dimensions before using them
@@ -278,9 +284,11 @@ class TerminalModel with ChangeNotifier {
     if (success) {
       _terminalOpened = true;
 
-      // On reconnect ("Reconnected to existing terminal"), server may replay recent output.
-      // If this TerminalView instance is reused (not rebuilt), duplicate lines can appear.
-      // We intentionally accept this tradeoff for now to keep logic simple.
+      // On reconnect, the server may replay recent output. That replay can include
+      // terminal queries like DSR/DA; xterm answers them through onOutput as
+      // "^[[1;1R^[[2;2R^[[>0;0;0c", which must not be sent back to the peer.
+      _suppressNextTerminalDataOutput =
+          message == 'Reconnected to existing terminal with pending output';
 
       // Fallback: if terminal view is not yet ready but already has valid
       // dimensions (e.g. layout completed before open response arrived),
@@ -339,6 +347,8 @@ class TerminalModel with ChangeNotifier {
     final data = evt['data'];
 
     if (data != null) {
+      final suppressTerminalOutput = _suppressNextTerminalDataOutput;
+      _suppressNextTerminalDataOutput = false;
       try {
         String text = '';
         if (data is String) {
@@ -358,7 +368,7 @@ class TerminalModel with ChangeNotifier {
           return;
         }
 
-        _writeToTerminal(text);
+        _writeToTerminal(text, suppressTerminalOutput: suppressTerminalOutput);
       } catch (e) {
         debugPrint('[TerminalModel] Failed to process terminal data: $e');
       }
@@ -368,7 +378,10 @@ class TerminalModel with ChangeNotifier {
   /// Write text to terminal, buffering if the view is not yet ready.
   /// All terminal output should go through this method to avoid NaN errors
   /// from writing before the terminal view has valid layout dimensions.
-  void _writeToTerminal(String text) {
+  void _writeToTerminal(
+    String text, {
+    bool suppressTerminalOutput = false,
+  }) {
     if (!_terminalViewReady) {
       // If a single chunk exceeds the cap, keep only its tail.
       // Note: truncation may split a multi-byte ANSI escape sequence,
@@ -380,33 +393,59 @@ class TerminalModel with ChangeNotifier {
         _pendingOutputChunks
           ..clear()
           ..add(truncated);
+        _pendingOutputSuppressFlags
+          ..clear()
+          ..add(suppressTerminalOutput);
         _pendingOutputSize = truncated.length;
       } else {
         _pendingOutputChunks.add(text);
+        _pendingOutputSuppressFlags.add(suppressTerminalOutput);
         _pendingOutputSize += text.length;
         // Drop oldest chunks if exceeds limit (whole chunks to preserve ANSI sequences)
         while (_pendingOutputSize > _kMaxOutputBufferChars &&
             _pendingOutputChunks.length > 1) {
           final removed = _pendingOutputChunks.removeAt(0);
+          _pendingOutputSuppressFlags.removeAt(0);
           _pendingOutputSize -= removed.length;
         }
       }
       return;
     }
-    terminal.write(text);
+    _writeTerminalChunk(text, suppressTerminalOutput: suppressTerminalOutput);
   }
 
   void _flushOutputBuffer() {
     if (_pendingOutputChunks.isEmpty) return;
     debugPrint(
         '[TerminalModel] Flushing $_pendingOutputSize buffered chars (${_pendingOutputChunks.length} chunks)');
-    for (final chunk in _pendingOutputChunks) {
-      terminal.write(chunk);
+    for (var i = 0; i < _pendingOutputChunks.length; i++) {
+      _writeTerminalChunk(
+        _pendingOutputChunks[i],
+        suppressTerminalOutput: _pendingOutputSuppressFlags[i],
+      );
     }
     _pendingOutputChunks.clear();
+    _pendingOutputSuppressFlags.clear();
     _pendingOutputSize = 0;
   }
 
+  void _writeTerminalChunk(
+    String text, {
+    required bool suppressTerminalOutput,
+  }) {
+    if (!suppressTerminalOutput) {
+      terminal.write(text);
+      return;
+    }
+    final previous = _suppressTerminalOutput;
+    _suppressTerminalOutput = true;
+    try {
+      terminal.write(text);
+    } finally {
+      _suppressTerminalOutput = previous;
+    }
+  }
+
   /// Mark terminal view as ready and flush buffered output.
   void _markViewReady() {
     if (_terminalViewReady) return;
@@ -433,7 +472,9 @@ class TerminalModel with ChangeNotifier {
     // Clear buffers to free memory
     _inputBuffer.clear();
     _pendingOutputChunks.clear();
+    _pendingOutputSuppressFlags.clear();
     _pendingOutputSize = 0;
+    _suppressNextTerminalDataOutput = false;
     // Terminal cleanup is handled server-side when service closes
     super.dispose();
   }