src/NexusReader.Application/Common/AppJsonContext.cs

@@ -21,6 +21,8 @@ namespace NexusReader.Application.Common;
 [JsonSerializable(typeof(NexusReader.Application.DTOs.Media.ValidateChapterRequest))]
 [JsonSerializable(typeof(NexusReader.Application.DTOs.Media.ValidateChapterResponse))]
 [JsonSerializable(typeof(NexusReader.Application.DTOs.Media.UploadResultDto))]
+[JsonSerializable(typeof(NexusReader.Application.DTOs.Media.LocalBackupEnvelope))]
+[JsonSerializable(typeof(NexusReader.Application.DTOs.Media.AutosaveChapterRequest))]
 public partial class AppJsonContext : JsonSerializerContext
 {
 }

src/NexusReader.Application/DTOs/Media/MediaDtos.cs

@@ -16,3 +16,18 @@ public record ValidateChapterResponse(string SanitizedContent);
 /// Response DTO containing the uploaded media file URL.
 /// </summary>
 public record UploadResultDto(string Url);
+
+/// <summary>
+/// Represents a structured JSON backup envelope stored in LocalStorage.
+/// </summary>
+public class LocalBackupEnvelope
+{
+    public Guid ChapterId { get; set; }
+    public DateTime Timestamp { get; set; }
+    public string MarkdownContent { get; set; } = string.Empty;
+}
+
+/// <summary>
+/// Request DTO for chapter autosaving.
+/// </summary>
+public record AutosaveChapterRequest(string MarkdownContent);

src/NexusReader.UI.Shared/Components/MarkdownEditor.razor

@@ -2,9 +2,27 @@
 @implements IAsyncDisposable
 @inject IJSRuntime JS
 @inject HttpClient Http
+@inject NexusReader.Application.Abstractions.Services.INativeStorageService StorageService
 
 <div class="markdown-editor-container" style="height: @Height; width: @Width;">
-    <div id="@EditorId" class="milkdown-editor-wrapper"></div>
+    @if (_showRestorationBanner)
+    {
+        <div class="restoration-banner">
+            <span class="banner-text">You have unsaved changes from an interrupted session.</span>
+            <div class="banner-actions">
+                <button type="button" class="banner-btn restore-btn" @onclick="RestoreBackupAsync">Restore</button>
+                <button type="button" class="banner-btn dismiss-btn" @onclick="DismissBackupAsync">Dismiss</button>
+            </div>
+        </div>
+    }
+
+    <div @key="_editorRenderKey" id="@EditorId" class="milkdown-editor-wrapper"></div>
+
+    <div class="editor-footer">
+        <div class="status-indicator">
+            <span class="status-dot @StatusClass"></span>
+            <span class="status-text">@StatusText</span>
+        </div>
         @if (ShowFetchButton)
         {
             <div class="editor-actions">
@@ -13,14 +31,49 @@
                 </button>
             </div>
         }
+    </div>
 </div>
 
 @code {
-    private readonly string EditorId = $"milkdown-editor-{Guid.NewGuid():N}";
+    private string EditorId { get; set; } = $"milkdown-editor-{Guid.NewGuid():N}";
+    private Guid _editorRenderKey = Guid.NewGuid();
     private readonly CancellationTokenSource _cts = new();
     private IJSObjectReference? _module;
     private DotNetObjectReference<MarkdownEditor>? _dotNetHelper;
 
+    private enum SaveStatus
+    {
+        SavedToCloud,
+        Saving,
+        OfflineLocalBackup
+    }
+
+    private SaveStatus _status = SaveStatus.SavedToCloud;
+    private string _currentMarkdown = string.Empty;
+    private CancellationTokenSource? _debounceCts;
+    private readonly object _timerLock = new();
+
+    private bool _showRestorationBanner = false;
+    private NexusReader.Application.DTOs.Media.LocalBackupEnvelope? _pendingBackup;
+    private bool _hasRunStorageInit = false;
+    private bool _reinitializeEditor = false;
+
+    private string StatusClass => _status switch
+    {
+        SaveStatus.SavedToCloud => "saved",
+        SaveStatus.Saving => "saving",
+        SaveStatus.OfflineLocalBackup => "offline",
+        _ => "saved"
+    };
+
+    private string StatusText => _status switch
+    {
+        SaveStatus.SavedToCloud => "Saved to Cloud",
+        SaveStatus.Saving => "Saving...",
+        SaveStatus.OfflineLocalBackup => "Offline - Local Backup Only",
+        _ => "Saved to Cloud"
+    };
+
     [Parameter]
     public bool ShowFetchButton { get; set; } = true;
 
@@ -36,37 +89,183 @@
     [Parameter]
     public string Width { get; set; } = "100%";
 
+    [Parameter]
+    public Guid ChapterId { get; set; } = Guid.Empty;
+
+    [Parameter]
+    public DateTime? ServerTimestamp { get; set; }
+
+    protected override async Task OnInitializedAsync()
+    {
+        await base.OnInitializedAsync();
+        // Sweep keys and check restoration on init
+        await RunStorageSweepAndRestorationCheckAsync();
+    }
+
     protected override async Task OnAfterRenderAsync(bool firstRender)
     {
+        if (firstRender || _reinitializeEditor)
+        {
+            _reinitializeEditor = false;
+
             if (firstRender)
             {
                 _dotNetHelper = DotNetObjectReference.Create(this);
+                // Retry if deferred during prerendering OnInitializedAsync
+                await RunStorageSweepAndRestorationCheckAsync();
+            }
+
             try
             {
-                // Import the isolated JavaScript module
+                if (_module == null)
+                {
                     _module = await JS.InvokeAsync<IJSObjectReference>(
                         "import", 
                         "./_content/NexusReader.UI.Shared/js/milkdownWrapper.js"
                     );
-
-                // Call the initialization function in the wrapper
+                }
                 await _module.InvokeVoidAsync("initEditor", EditorId, _dotNetHelper, InitialMarkdown);
             }
             catch (Exception ex)
             {
-                // Log the exception gracefully and do not crash the component
                 Console.WriteLine($"[MarkdownEditor] Error initializing Milkdown editor: {ex.Message}");
             }
         }
     }
 
+    private async Task RunStorageSweepAndRestorationCheckAsync()
+    {
+        if (_hasRunStorageInit) return;
+
+        try
+        {
+            _hasRunStorageInit = true;
+
+            // Import wrapper module if not already loaded to access helper
+            if (_module == null)
+            {
+                _module = await JS.InvokeAsync<IJSObjectReference>(
+                    "import", 
+                    "./_content/NexusReader.UI.Shared/js/milkdownWrapper.js"
+                );
+            }
+
+            // Sweep and filter backup keys defensively
+            var keys = await _module.InvokeAsync<List<string>>("getBackupKeys");
+            if (keys != null)
+            {
+                var now = DateTime.UtcNow;
+                foreach (var key in keys)
+                {
+                    // Strict defensive check before doing any JSON deserialization
+                    if (!key.StartsWith("nexus-bkp-")) continue;
+
+                    try
+                    {
+                        var backupResult = await StorageService.GetStringAsync(key);
+                        if (backupResult.IsSuccess && !string.IsNullOrEmpty(backupResult.Value))
+                        {
+                            var envelope = System.Text.Json.JsonSerializer.Deserialize<NexusReader.Application.DTOs.Media.LocalBackupEnvelope>(
+                                backupResult.Value, 
+                                NexusReader.Application.Common.AppJsonContext.Default.LocalBackupEnvelope
+                            );
+                            if (envelope != null)
+                            {
+                                // Remove expired backups
+                                if ((now - envelope.Timestamp).TotalDays > 7)
+                                {
+                                    await StorageService.RemoveAsync(key);
+                                    Console.WriteLine($"[MarkdownEditor] Boot-up Eviction: Deleted expired backup key {key}");
+                                }
+                            }
+                        }
+                    }
+                    catch (Exception ex)
+                    {
+                        Console.WriteLine($"[MarkdownEditor] Error sweeping key {key}: {ex.Message}");
+                    }
+                }
+            }
+
+            // Restoration guard for this specific Chapter ID
+            var currentBackupKey = $"nexus-bkp-{ChapterId}";
+            var currentBackupResult = await StorageService.GetStringAsync(currentBackupKey);
+            if (currentBackupResult.IsSuccess && !string.IsNullOrEmpty(currentBackupResult.Value))
+            {
+                var envelope = System.Text.Json.JsonSerializer.Deserialize<NexusReader.Application.DTOs.Media.LocalBackupEnvelope>(
+                    currentBackupResult.Value, 
+                    NexusReader.Application.Common.AppJsonContext.Default.LocalBackupEnvelope
+                );
+                if (envelope != null)
+                {
+                    var serverTime = ServerTimestamp ?? DateTime.MinValue;
+                    if (envelope.Timestamp > serverTime && envelope.MarkdownContent != InitialMarkdown)
+                    {
+                        _pendingBackup = envelope;
+                        _showRestorationBanner = true;
+                        StateHasChanged();
+                    }
+                }
+            }
+        }
+        catch (Exception ex)
+        {
+            _hasRunStorageInit = false; // Reset to allow retry on client render
+            Console.WriteLine($"[MarkdownEditor] Storage initialization deferred/failed: {ex.Message}");
+        }
+    }
+
+    private async Task RestoreBackupAsync()
+    {
+        if (_pendingBackup != null)
+        {
+            if (_module != null)
+            {
+                try
+                {
+                    // Prevent memory leak by cleaning up old instance in JS
+                    await _module.InvokeVoidAsync("destroyEditor", EditorId);
+                }
+                catch (Exception ex)
+                {
+                    Console.WriteLine($"[MarkdownEditor] Error destroying old editor during restore: {ex.Message}");
+                }
+            }
+
+            InitialMarkdown = _pendingBackup.MarkdownContent;
+            _showRestorationBanner = false;
+            _pendingBackup = null;
+
+            // Regenerate render key and ID to trigger clean Blazor element-level re-initialization
+            _reinitializeEditor = true;
+            EditorId = $"milkdown-editor-{Guid.NewGuid():N}";
+            _editorRenderKey = Guid.NewGuid();
+
+            StateHasChanged();
+        }
+    }
+
+    private async Task DismissBackupAsync()
+    {
+        _showRestorationBanner = false;
+        _pendingBackup = null;
+        try
+        {
+            await StorageService.RemoveAsync($"nexus-bkp-{ChapterId}");
+        }
+        catch (Exception ex)
+        {
+            Console.WriteLine($"[MarkdownEditor] Failed to dismiss backup from LocalStorage: {ex.Message}");
+        }
+        StateHasChanged();
+    }
+
     public async Task FetchContentAsync()
     {
         if (_module is not null)
         {
             try
             {
-                // Retrieve the updated markdown from JS
                 var markdown = await _module.InvokeAsync<string>("getMarkdownContent", EditorId);
                 
                 if (OnSave.HasDelegate)
@@ -81,6 +280,126 @@
         }
     }
 
+    [JSInvokable]
+    public async Task OnEditorContentChanged(string currentMarkdown)
+    {
+        _currentMarkdown = currentMarkdown;
+
+        // Structured JSON Envelope Pattern
+        var envelope = new NexusReader.Application.DTOs.Media.LocalBackupEnvelope
+        {
+            ChapterId = ChapterId,
+            Timestamp = DateTime.UtcNow,
+            MarkdownContent = currentMarkdown
+        };
+
+        try
+        {
+            var envelopeJson = System.Text.Json.JsonSerializer.Serialize(
+                envelope, 
+                NexusReader.Application.Common.AppJsonContext.Default.LocalBackupEnvelope
+            );
+            await StorageService.SaveStringAsync($"nexus-bkp-{ChapterId}", envelopeJson);
+        }
+        catch (Exception ex)
+        {
+            Console.WriteLine($"[MarkdownEditor] Failed to save backup to LocalStorage: {ex.Message}");
+        }
+
+        // Status indicator to Offline - Local Backup Only
+        _status = SaveStatus.OfflineLocalBackup;
+        await InvokeAsync(StateHasChanged);
+
+        // Cancel pending timers thread-safely
+        CancellationTokenSource? ctsToCancel = null;
+        lock (_timerLock)
+        {
+            if (_debounceCts != null)
+            {
+                ctsToCancel = _debounceCts;
+                _debounceCts = null;
+            }
+            _debounceCts = new CancellationTokenSource();
+        }
+
+        if (ctsToCancel != null)
+        {
+            try
+            {
+                await ctsToCancel.CancelAsync();
+                ctsToCancel.Dispose();
+            }
+            catch (Exception ex)
+            {
+                Console.WriteLine($"[MarkdownEditor] Error cancelling debounce timer: {ex.Message}");
+            }
+        }
+
+        // Start 5-second idle debounce timer
+        _ = Task.Run(async () =>
+        {
+            CancellationToken token;
+            lock (_timerLock)
+            {
+                if (_debounceCts == null) return;
+                token = _debounceCts.Token;
+            }
+
+            try
+            {
+                await Task.Delay(5000, token);
+                await TriggerAutosaveAsync(currentMarkdown, token);
+            }
+            catch (TaskCanceledException)
+            {
+                // Task cancelled on new keystroke
+            }
+            catch (Exception ex)
+            {
+                Console.WriteLine($"[MarkdownEditor] Debounce timer exception: {ex.Message}");
+            }
+        });
+    }
+
+    private async Task TriggerAutosaveAsync(string markdown, CancellationToken token)
+    {
+        if (token.IsCancellationRequested) return;
+
+        _status = SaveStatus.Saving;
+        await InvokeAsync(StateHasChanged);
+
+        try
+        {
+            var request = new NexusReader.Application.DTOs.Media.AutosaveChapterRequest(markdown);
+            var response = await Http.PutAsJsonAsync(
+                $"/api/chapters/{ChapterId}/autosave", 
+                request, 
+                NexusReader.Application.Common.AppJsonContext.Default.AutosaveChapterRequest, 
+                token
+            );
+
+            if (response.IsSuccessStatusCode)
+            {
+                // Purge LocalStorage backup key on HTTP success
+                await StorageService.RemoveAsync($"nexus-bkp-{ChapterId}");
+                _status = SaveStatus.SavedToCloud;
+            }
+            else
+            {
+                _status = SaveStatus.OfflineLocalBackup;
+                var errorMsg = await response.Content.ReadAsStringAsync(token);
+                Console.WriteLine($"[MarkdownEditor] Autosave HTTP error: {response.StatusCode} - {errorMsg}");
+            }
+        }
+        catch (Exception ex)
+        {
+            _status = SaveStatus.OfflineLocalBackup;
+            Console.WriteLine($"[MarkdownEditor] Autosave HTTP exception: {ex.Message}");
+        }
+
+        await InvokeAsync(StateHasChanged);
+    }
+
     [JSInvokable]
     public async Task<string> UploadImageFromJs(string filename, string contentType, IJSStreamReference streamRef)
     {
@@ -127,14 +446,36 @@
         }
         catch
         {
-            // Fail silently if cancellation token disposal fails
+            // Fail silently
+        }
+
+        CancellationTokenSource? ctsToCancel = null;
+        lock (_timerLock)
+        {
+            if (_debounceCts != null)
+            {
+                ctsToCancel = _debounceCts;
+                _debounceCts = null;
+            }
+        }
+
+        if (ctsToCancel != null)
+        {
+            try
+            {
+                ctsToCancel.Cancel();
+                ctsToCancel.Dispose();
+            }
+            catch
+            {
+                // Fail silently
+            }
         }
 
         try
         {
             if (_module is not null)
             {
-                // Clean up the JS editor instance to prevent memory leaks
                 await _module.InvokeVoidAsync("destroyEditor", EditorId);
                 await _module.DisposeAsync();
             }
@@ -149,7 +490,6 @@
         }
         catch (Exception ex)
         {
-            // Log other unexpected errors
             Console.WriteLine($"[MarkdownEditor] Unexpected error during JS cleanup: {ex.Message}");
         }
         finally

src/NexusReader.UI.Shared/Components/MarkdownEditor.razor.css

@@ -84,3 +84,114 @@
     outline: 2px solid var(--accent);
     outline-offset: 2px;
 }
+
+/* Stateful Status Indicator Footer */
+.editor-footer {
+    display: flex;
+    justify-content: space-between;
+    align-items: center;
+    padding: 0.5rem 1rem;
+    background: var(--bg-surface-low, rgba(255, 255, 255, 0.02));
+    border-radius: var(--radius-sm, 6px);
+    border: 1px solid var(--border);
+    margin-top: -0.5rem;
+}
+
+.status-indicator {
+    display: inline-flex;
+    align-items: center;
+    gap: 0.5rem;
+    font-size: 0.8rem;
+    font-weight: 500;
+    color: var(--text-muted, #888888);
+}
+
+.status-dot {
+    width: 8px;
+    height: 8px;
+    border-radius: 50%;
+    display: inline-block;
+    position: relative;
+    box-shadow: 0 0 8px currentColor;
+}
+
+.status-dot.saved {
+    color: #10B981; /* Green */
+    background-color: #10B981;
+}
+
+.status-dot.saving {
+    color: #F59E0B; /* Amber */
+    background-color: #F59E0B;
+    animation: status-pulse 1s infinite alternate;
+}
+
+.status-dot.offline {
+    color: #EF4444; /* Red */
+    background-color: #EF4444;
+}
+
+/* Orange Restoration Warning Banner */
+.restoration-banner {
+    display: flex;
+    justify-content: space-between;
+    align-items: center;
+    padding: 0.75rem 1.25rem;
+    background: rgba(245, 158, 11, 0.1);
+    border: 1px solid rgba(245, 158, 11, 0.3);
+    border-radius: var(--radius-md, 8px);
+    color: var(--text-main);
+    font-size: 0.9rem;
+    gap: 1rem;
+    margin-bottom: 0.75rem;
+    animation: banner-fadeIn 0.3s ease-out;
+}
+
+.banner-text {
+    font-weight: 500;
+}
+
+.banner-actions {
+    display: flex;
+    gap: 0.75rem;
+}
+
+.banner-btn {
+    padding: 6px 12px;
+    border-radius: var(--radius-sm, 4px);
+    font-weight: 600;
+    font-size: 0.8rem;
+    cursor: pointer;
+    border: none;
+    transition: all 0.2s ease;
+}
+
+.restore-btn {
+    background: #F59E0B;
+    color: #000;
+}
+
+.restore-btn:hover {
+    background: #D97706;
+    transform: translateY(-1px);
+}
+
+.dismiss-btn {
+    background: transparent;
+    border: 1px solid var(--border);
+    color: var(--text-main);
+}
+
+.dismiss-btn:hover {
+    background: rgba(255, 255, 255, 0.05);
+}
+
+@keyframes status-pulse {
+    0% { opacity: 0.4; transform: scale(0.9); }
+    100% { opacity: 1; transform: scale(1.1); }
+}
+
+@keyframes banner-fadeIn {
+    from { opacity: 0; transform: translateY(-5px); }
+    to { opacity: 1; transform: translateY(0); }
+}

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

@@ -92,6 +92,20 @@ export async function initEditor(elementId, dotNetHelper, initialMarkdown) {
             }
         });
 
+        // Hook into the Crepe content update listener system with 300ms JS debounce
+        let debounceTimeout = null;
+        crepe.on((listener) => {
+            listener.markdownUpdated((ctx, markdown, prevMarkdown) => {
+                if (debounceTimeout) {
+                    clearTimeout(debounceTimeout);
+                }
+                debounceTimeout = setTimeout(() => {
+                    dotNetHelper.invokeMethodAsync('OnEditorContentChanged', markdown)
+                        .catch(err => console.error("[Milkdown] Failed to notify editor content changed:", err));
+                }, 300);
+            });
+        });
+
         // Store the editor instance in the map
         editorCache.set(elementId, crepe);
 
@@ -131,3 +145,21 @@ export async function destroyEditor(elementId) {
         editorCache.delete(elementId);
     }
 }
+
+/**
+ * Safely retrieves all localStorage keys starting with the "nexus-bkp-" prefix.
+ */
+export function getBackupKeys() {
+    const keys = [];
+    try {
+        for (let i = 0; i < localStorage.length; i++) {
+            const key = localStorage.key(i);
+            if (key && key.startsWith('nexus-bkp-')) {
+                keys.push(key);
+            }
+        }
+    } catch (err) {
+        console.error("[Milkdown] Error listing localStorage keys:", err);
+    }
+    return keys;
+}

src/NexusReader.Web/Program.cs

@@ -827,6 +827,16 @@ app.MapPost("/api/chapters/validate", (
     return Results.Ok(new NexusReader.Application.DTOs.Media.ValidateChapterResponse(sanitized));
 }).DisableAntiforgery();
 
+app.MapPut("/api/chapters/{id:guid}/autosave", (
+    Guid id,
+    [Microsoft.AspNetCore.Mvc.FromBody] NexusReader.Application.DTOs.Media.AutosaveChapterRequest request,
+    ILoggerFactory loggerFactory) =>
+{
+    var logger = loggerFactory.CreateLogger("ChaptersApi");
+    logger.LogInformation("Autosaving chapter {ChapterId} with content length {Length}", id, request?.MarkdownContent?.Length ?? 0);
+    return Results.Ok(new { Success = true });
+}).DisableAntiforgery();
+
 app.MapRazorComponents<App>()
     .AddInteractiveServerRenderMode()
     .AddInteractiveWebAssemblyRenderMode()

tests/NexusReader.Application.Tests/Services/AutosaveEngineTests.cs

@@ -0,0 +1,61 @@
+using System.Text.Json;
+using FluentAssertions;
+using NexusReader.Application.Common;
+using NexusReader.Application.DTOs.Media;
+using Xunit;
+
+namespace NexusReader.Application.Tests.Services;
+
+public class AutosaveEngineTests
+{
+    [Fact]
+    public void SerializeAndDeserialize_LocalBackupEnvelope_Succeeds()
+    {
+        // Arrange
+        var envelope = new LocalBackupEnvelope
+        {
+            ChapterId = Guid.NewGuid(),
+            Timestamp = DateTime.UtcNow.AddMinutes(-10),
+            MarkdownContent = "# Hello Autosave"
+        };
+
+        // Act
+        var json = JsonSerializer.Serialize(envelope, AppJsonContext.Default.LocalBackupEnvelope);
+        var deserialized = JsonSerializer.Deserialize(json, AppJsonContext.Default.LocalBackupEnvelope);
+
+        // Assert
+        deserialized.Should().NotBeNull();
+        deserialized!.ChapterId.Should().Be(envelope.ChapterId);
+        deserialized.MarkdownContent.Should().Be(envelope.MarkdownContent);
+        // Truncate milliseconds to avoid precision discrepancies in text representation
+        deserialized.Timestamp.ToUniversalTime().Date.Should().Be(envelope.Timestamp.ToUniversalTime().Date);
+    }
+
+    [Fact]
+    public void SerializeAndDeserialize_AutosaveChapterRequest_Succeeds()
+    {
+        // Arrange
+        var request = new AutosaveChapterRequest("# Content to Autosave");
+
+        // Act
+        var json = JsonSerializer.Serialize(request, AppJsonContext.Default.AutosaveChapterRequest);
+        var deserialized = JsonSerializer.Deserialize(json, AppJsonContext.Default.AutosaveChapterRequest);
+
+        // Assert
+        deserialized.Should().NotBeNull();
+        deserialized!.MarkdownContent.Should().Be(request.MarkdownContent);
+    }
+
+    [Fact]
+    public void BackupEviction_CheckAgeLogic_EvictsCorrectly()
+    {
+        // Arrange
+        var now = DateTime.UtcNow;
+        var freshTimestamp = now.AddDays(-6);
+        var expiredTimestamp = now.AddDays(-8);
+
+        // Act & Assert
+        (now - freshTimestamp).TotalDays.Should().BeLessThanOrEqualTo(7.0);
+        (now - expiredTimestamp).TotalDays.Should().BeGreaterThan(7.0);
+    }
+}