Fix preview generation race condition when toggling filters rapidly

Concurrent _generateProcessedPreview() calls (fired from non-awaited timer
callbacks) would clobber each other's cancel tokens and prematurely clear
the isGeneratingPreview flag, causing previews to silently stop updating.

Add a generation counter so only the most recent call updates shared state,
use a local cancel token reference instead of the instance variable, and
guard _previewProcess cleanup to avoid nulling a newer call's process.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: StuartCameronCode

app/lib/services/preview_generator.dart

@@ -213,6 +213,7 @@ class PreviewGenerator {
     // We no longer double for FPSDivisor=1 because we're seeking in the source
     final frameNumber = (timeSeconds * _frameRate).round();
     final configPath = '$_tempDir/preview_config_${DateTime.now().millisecondsSinceEpoch}.json';
+    Process? process;
 
     try {
       // Set TFF based on field order for QTGMC
@@ -250,7 +251,7 @@ class PreviewGenerator {
       // Run worker in preview mode
       // Use local variable to avoid race conditions when another preview request cancels this one
       // Set workingDirectory to the worker's parent directory so relative deps paths resolve correctly
-      final process = await Process.start(
+      process = await Process.start(
         _workerPath!,
         [
           '--config', configPath,
@@ -299,7 +300,9 @@ class PreviewGenerator {
 
       // Wait for process to complete (use local variable to avoid race conditions)
       final exitCode = await process.exitCode;
-      _previewProcess = null;
+      if (_previewProcess == process) {
+        _previewProcess = null;
+      }
 
       // Log the result
       _previewLog.add('[${DateTime.now().toIso8601String()}] Process exited with code $exitCode, output size: ${pngBytes.length} bytes');
@@ -321,7 +324,9 @@ class PreviewGenerator {
       _lastError = 'Preview generation error: $e';
       _previewLog.add('[ERROR] $e');
     } finally {
-      _previewProcess = null;
+      if (_previewProcess == process) {
+        _previewProcess = null;
+      }
       // Clean up config file on error
       try {
         await File(configPath).delete();

app/lib/viewmodels/main_viewmodel.dart

@@ -61,6 +61,7 @@ class MainViewModel extends ChangeNotifier {
   bool _isGeneratingPreview = false;
   CancelToken? _previewCancelToken;
   Timer? _previewDebounceTimer;
+  int _previewGeneration = 0;
   Timer? _zoomDebounceTimer;
   bool _isLoadingZoomedThumbnails = false;
 
@@ -785,17 +786,25 @@ class MainViewModel extends ChangeNotifier {
   }
 
   /// Generates the processed preview at the current scrubber position.
+  ///
+  /// Uses a generation counter to handle concurrent calls safely. The timer
+  /// callback cannot await this async method, so multiple calls can overlap
+  /// when filters are toggled rapidly. Only the most recent generation is
+  /// allowed to update shared state (_isGeneratingPreview, processedPreview).
   Future<void> _generateProcessedPreview() async {
     final item = selectedItem;
     if (item == null) return;
 
     // Cancel any existing preview generation
     _cancelPreviewGeneration();
 
+    final generation = ++_previewGeneration;
+
     _isGeneratingPreview = true;
     notifyListeners();
 
-    _previewCancelToken = CancelToken();
+    final cancelToken = CancelToken();
+    _previewCancelToken = cancelToken;
     final timeSeconds = item.scrubberPosition * _previewGenerator.duration;
 
     try {
@@ -804,18 +813,22 @@ class MainViewModel extends ChangeNotifier {
         pipeline: _processingPipeline,
         fieldOrder: effectiveFieldOrder,
         encodingSettings: _encodingSettings,
-        cancelToken: _previewCancelToken,
+        cancelToken: cancelToken,
       );
 
-      if (!(_previewCancelToken?.isCancelled ?? true)) {
+      if (generation == _previewGeneration && !cancelToken.isCancelled) {
         item.processedPreview = preview;
       }
     } catch (e) {
       // Ignore errors from cancelled previews
     }
 
-    _isGeneratingPreview = false;
-    notifyListeners();
+    // Only clear the generating flag if this is still the latest generation.
+    // Otherwise a newer call is in flight and owns the flag.
+    if (generation == _previewGeneration) {
+      _isGeneratingPreview = false;
+      notifyListeners();
+    }
   }
 
   /// Cancels any ongoing preview generation.