ux(reader): implement scroll-to-top on chapter switch and cancel stale background AI tasks

src/NexusReader.UI.Shared/Components/Organisms/ReaderCanvas.razor

@@ -308,6 +308,11 @@
         StatusMessage = "Wczytywanie treści...";
         StateHasChanged();
 
+        if (string.IsNullOrEmpty(NavigationService.PendingScrollBlockId))
+        {
+            await ScrollToTopAsync();
+        }
+
         var authState = await AuthStateProvider.GetAuthenticationStateAsync();
         var userId = authState.User.FindFirst(System.Security.Claims.ClaimTypes.NameIdentifier)?.Value;
 
@@ -374,6 +379,19 @@
         }
     }
 
+    public async Task ScrollToTopAsync()
+    {
+        try
+        {
+            var module = _viewportModule ?? await JS.InvokeAsync<IJSObjectReference>("import", "./_content/NexusReader.UI.Shared/js/viewport.js");
+            await module.InvokeVoidAsync("scrollToTop");
+        }
+        catch (Exception ex)
+        {
+            Logger.LogWarning(ex, "Failed to scroll reader canvas to top.");
+        }
+    }
+
     private Task HandleUpdate() => InvokeAsync(StateHasChanged);
 
     private void HandleEscape()

src/NexusReader.UI.Shared/Services/KnowledgeCoordinator.cs

@@ -17,6 +17,9 @@ public sealed partial class KnowledgeCoordinator : IDisposable
     private readonly IReaderInteractionService _interactionService;
     private readonly ILogger<KnowledgeCoordinator> _logger;
 
+    private CancellationTokenSource? _graphCts;
+    private CancellationTokenSource? _quizCts;
+    
     public string CurrentFullPageContent { get; private set; } = string.Empty;
     
     /// <summary>
@@ -77,6 +80,11 @@ public sealed partial class KnowledgeCoordinator : IDisposable
 
     public async Task ProcessFullPageAsync(string fullContent, string tenantId = "global", Guid? ebookId = null)
     {
+        _graphCts?.Cancel();
+        _graphCts?.Dispose();
+        _graphCts = new CancellationTokenSource();
+        var token = _graphCts.Token;
+
         if (string.IsNullOrWhiteSpace(fullContent)) return;
 
         CurrentFullPageContent = fullContent;
@@ -87,7 +95,9 @@ public sealed partial class KnowledgeCoordinator : IDisposable
         
         try
         {
-            var result = await _knowledgeService.GetGraphDataAsync(fullContent, tenantId, ebookId);
+            var result = await _knowledgeService.GetGraphDataAsync(fullContent, tenantId, ebookId, token);
+            token.ThrowIfCancellationRequested();
+
             if (result.IsSuccess)
             {
                 var packet = result.Value;
@@ -103,12 +113,19 @@ public sealed partial class KnowledgeCoordinator : IDisposable
             
             await _graphService.SetLoading(false);
         }
+        catch (OperationCanceledException)
+        {
+            _logger.LogInformation("[KnowledgeCoordinator] Graph generation task was canceled.");
+        }
         catch (Exception ex)
+        {
+            if (!token.IsCancellationRequested)
             {
                 await _graphService.SetLoading(false);
                 LogGraphError(ex, tenantId);
             }
         }
+    }
 
     public async Task OnBlockReachedAsync(string blockId, string content)
     {
@@ -118,11 +135,18 @@ public sealed partial class KnowledgeCoordinator : IDisposable
 
     public async Task<Result<KnowledgePacket>> RequestSummaryAndQuizAsync(string content, string tenantId = "global")
     {
+        _quizCts?.Cancel();
+        _quizCts?.Dispose();
+        _quizCts = new CancellationTokenSource();
+        var token = _quizCts.Token;
+
         await _quizService.SetHydrating(true);
         LogRequestingSummary(tenantId);
         try
         {
-            var result = await _knowledgeService.GetSummaryAndQuizAsync(content, tenantId);
+            var result = await _knowledgeService.GetSummaryAndQuizAsync(content, tenantId, cancellationToken: token);
+            token.ThrowIfCancellationRequested();
+
             if (result.IsSuccess)
             {
                 var packet = result.Value;
@@ -138,11 +162,20 @@ public sealed partial class KnowledgeCoordinator : IDisposable
             LogSummaryWarning(tenantId);
             return Result.Fail(result.Errors);
         }
+        catch (OperationCanceledException)
+        {
+            _logger.LogInformation("[KnowledgeCoordinator] Quiz and summary generation task was canceled.");
+            return Result.Fail("Task canceled");
+        }
         catch (Exception ex)
+        {
+            if (!token.IsCancellationRequested)
             {
                 LogSummaryError(ex, tenantId);
                 return Result.Fail(new Error("Error requesting summary and quiz").CausedBy(ex));
             }
+            return Result.Fail("Task canceled");
+        }
         finally
         {
             await _quizService.SetHydrating(false);
@@ -151,6 +184,14 @@ public sealed partial class KnowledgeCoordinator : IDisposable
 
     public async Task ClearAsync()
     {
+        _graphCts?.Cancel();
+        _graphCts?.Dispose();
+        _graphCts = null;
+
+        _quizCts?.Cancel();
+        _quizCts?.Dispose();
+        _quizCts = null;
+
         CurrentFullPageContent = string.Empty;
         await _graphService.Clear();
         await _quizService.SetQuiz(null, null);
@@ -159,6 +200,12 @@ public sealed partial class KnowledgeCoordinator : IDisposable
     public void Dispose()
     {
         _interactionService.OnNodeSelected -= HandleNodeSelected;
+
+        _graphCts?.Cancel();
+        _graphCts?.Dispose();
+
+        _quizCts?.Cancel();
+        _quizCts?.Dispose();
     }
 
     [LoggerMessage(Level = LogLevel.Information, Message = "[KnowledgeCoordinator] Generating full page graph for tenant: {TenantId}")]

src/NexusReader.UI.Shared/wwwroot/js/viewport.js

@@ -38,3 +38,13 @@ export function scrollIntoView(id) {
     }
     return false;
 }
+
+export function scrollToTop() {
+    const el = document.querySelector('.reader-canvas');
+    if (el) {
+        el.scrollTop = 0;
+        return true;
+    }
+    return false;
+}
+