Back button bug fix

Assets/Darkmatter/Code/Features/GameplayFlow/Systems/GameplayFlowController.cs

@@ -46,6 +46,7 @@ namespace Darkmatter.Features.GameplayFlow.Systems
         private string _templateId;
         private DrawingPhase _phase = DrawingPhase.ShapeBuilding;
         private bool _allContentReported;
+        private bool _navigating;
         private float _lastThumbnailTime = float.NegativeInfinity;
 
         private IDisposable _assembledSub;
@@ -165,6 +166,13 @@ namespace Darkmatter.Features.GameplayFlow.Systems
         }
 
         public async UniTask BackAsync(CancellationToken ct)
+        {
+            // A second navigation while one runs (e.g. Back pressed during the completion
+            // animation inside NextAsync) would interleave two saves and two Colorbook loads,
+            // and Clear() would rip the celebration apart mid-flight. First navigation wins.
+            if (_navigating) return;
+            _navigating = true;
+            try
             {
                 await SaveCurrentAsync(CancellationToken.None);
                 _loadingScreen.Show();
@@ -173,6 +181,11 @@ namespace Darkmatter.Features.GameplayFlow.Systems
                 await _scenes.LoadSceneAsync(nameof(GameScene.Colorbook), progress: null, cancellationToken: ct);
                 await _scenes.UnloadSceneAsync(nameof(GameScene.Gameplay), progress: null, cancellationToken: ct);
             }
+            finally
+            {
+                _navigating = false;
+            }
+        }
 
         public async UniTask SaveAsync(CancellationToken ct)
         {
@@ -183,6 +196,10 @@ namespace Darkmatter.Features.GameplayFlow.Systems
         }
 
         public async UniTask NextAsync(CancellationToken ct)
+        {
+            if (_navigating) return;
+            _navigating = true;
+            try
             {
                 // Drop any debounced autosave so it can't fire during/after the completion
                 // animation and capture the animation (or an empty paper) into the thumbnail.
@@ -210,23 +227,19 @@ namespace Darkmatter.Features.GameplayFlow.Systems
                 }
 
                 var nextId = _catalog.GetNextTemplate(_templateId);
-            if (string.IsNullOrEmpty(nextId))
-            {
-                _loadingScreen.Show();
-                _shapeBuilder.Clear();
-                _coloring.Clear();
-                await _scenes.LoadSceneAsync(nameof(GameScene.Colorbook), progress: null, cancellationToken: default);
-                await _scenes.UnloadSceneAsync(nameof(GameScene.Gameplay), progress: null, cancellationToken: default);
-                return;
-            }
-
                 _loadingScreen.Show();
                 _shapeBuilder.Clear();
                 _coloring.Clear();
                 await _scenes.LoadSceneAsync(nameof(GameScene.Colorbook), progress: null, cancellationToken: default);
                 await _scenes.UnloadSceneAsync(nameof(GameScene.Gameplay), progress: null, cancellationToken: default);
+                if (!string.IsNullOrEmpty(nextId))
                     _bus.Publish(new DrawingSelectedSignal(nextId));
             }
+            finally
+            {
+                _navigating = false;
+            }
+        }
         
         
         private void OnShapeAssembled(ShapeAssembledSignal signal)
@@ -269,6 +282,10 @@ namespace Darkmatter.Features.GameplayFlow.Systems
         }
 
         private async UniTaskVoid EditAsync(string newTemplateId)
+        {
+            if (_navigating) return;
+            _navigating = true;
+            try
             {
                 await SaveCurrentAsync(CancellationToken.None);
                 _loadingScreen.Show();
@@ -278,6 +295,11 @@ namespace Darkmatter.Features.GameplayFlow.Systems
                 await _scenes.UnloadSceneAsync(nameof(GameScene.Gameplay), progress: null, cancellationToken: default);
                 _bus.Publish(new DrawingSelectedSignal(newTemplateId));
             }
+            finally
+            {
+                _navigating = false;
+            }
+        }
 
         private async UniTaskVoid DebouncedAutosaveAsync(CancellationToken ct)
         {