diff --git a/YKanBan.Tests/Storage/AppData/AppConfigStoreTests.cs b/YKanBan.Tests/Storage/AppData/AppConfigStoreTests.cs
index 74d832d..c04a37f 100644
--- a/YKanBan.Tests/Storage/AppData/AppConfigStoreTests.cs
+++ b/YKanBan.Tests/Storage/AppData/AppConfigStoreTests.cs
@@ -289,7 +289,7 @@ public class AppConfigStoreTests
 
         AppConfig config = store.Load();
 
-        Assert.IsFalse(store.SaveIfChanged(config));
+        Assert.IsFalse(store.SaveIfDirty(config));
         Assert.IsFalse(File.Exists(path));
     }
 
@@ -304,7 +304,7 @@ public class AppConfigStoreTests
 
         AppConfig config = store.Load();
 
-        Assert.IsFalse(store.SaveIfChanged(config));
+        Assert.IsFalse(store.SaveIfDirty(config));
         Assert.AreEqual(original, File.ReadAllText(path));
     }
 
@@ -318,15 +318,15 @@ public class AppConfigStoreTests
 
         config.Sort.Card = CardSortOption.UpdatedAt;
 
-        Assert.IsTrue(store.SaveIfChanged(config));
+        Assert.IsTrue(store.SaveIfDirty(config));
         Assert.AreEqual(CardSortOption.UpdatedAt, new AppConfigStore(path).Load().Sort.Card);
 
         // Saved state is the new baseline.
-        Assert.IsFalse(store.SaveIfChanged(config));
+        Assert.IsFalse(store.SaveIfDirty(config));
     }
 
     [TestMethod]
-    public void ChangeRevertedWithinSessionIsNotWritten()
+    public void ChangeRevertedWithinSessionIsStillWritten()
     {
         using var directory = new TempDirectory();
         string path = Path.Combine(directory.FullPath, "app.json");
@@ -336,7 +336,49 @@ public class AppConfigStoreTests
         config.Theme = ThemeOption.Dark;
         config.Theme = ThemeOption.FollowSystem;
 
-        Assert.IsFalse(store.SaveIfChanged(config));
+        Assert.IsTrue(store.SaveIfDirty(config));
+        Assert.IsTrue(File.Exists(path));
+    }
+
+    [TestMethod]
+    public void AssigningTheCurrentValuesIsNotAChange()
+    {
+        using var directory = new TempDirectory();
+        string path = Path.Combine(directory.FullPath, "app.json");
+        var store = new AppConfigStore(path);
+        AppConfig config = store.Load();
+
+        config.Theme = config.Theme;
+        config.Sort.Card = config.Sort.Card;
+        config.Confirmations.DeleteTag = config.Confirmations.DeleteTag;
+
+        Assert.IsFalse(store.SaveIfDirty(config));
         Assert.IsFalse(File.Exists(path));
     }
+
+    [TestMethod]
+    public void NestedSettingChangeMarksTheConfigurationDirty()
+    {
+        using var directory = new TempDirectory();
+        string path = Path.Combine(directory.FullPath, "app.json");
+        var store = new AppConfigStore(path);
+        AppConfig config = store.Load();
+
+        config.Confirmations.DiscardEdit = false;
+
+        Assert.IsTrue(config.IsDirty);
+        Assert.IsTrue(store.SaveIfDirty(config));
+        Assert.IsFalse(config.IsDirty);
+        Assert.IsFalse(new AppConfigStore(path).Load().Confirmations.DiscardEdit);
+    }
+
+    [TestMethod]
+    public void LoadedFileIsNotDirty()
+    {
+        using var directory = new TempDirectory();
+        string path = Path.Combine(directory.FullPath, "app.json");
+        File.WriteAllText(path, """{ "theme": "dark", "confirmations": { "delete-card": false } }""");
+
+        Assert.IsFalse(new AppConfigStore(path).Load().IsDirty);
+    }
 }
diff --git a/YKanBan/Services/AppServices.cs b/YKanBan/Services/AppServices.cs
index 3e719e8..fa994ab 100644
--- a/YKanBan/Services/AppServices.cs
+++ b/YKanBan/Services/AppServices.cs
@@ -82,7 +82,7 @@ public sealed class AppServices : IDisposable
     {
         try
         {
-            ConfigStore.SaveIfChanged(Config);
+            ConfigStore.SaveIfDirty(Config);
         }
         catch (Exception exception) when (exception is IOException or UnauthorizedAccessException)
         {
diff --git a/YKanBan/Storage/AppData/AppConfig.cs b/YKanBan/Storage/AppData/AppConfig.cs
index ec52fc4..71aa342 100644
--- a/YKanBan/Storage/AppData/AppConfig.cs
+++ b/YKanBan/Storage/AppData/AppConfig.cs
@@ -62,65 +62,101 @@ public enum ThemeOption
     FollowSystem,
 }
 
+/// <summary>
+/// Base of the app.json sections: records whether any setting was changed
+/// since the last load or save (the dirty flag). Assigning the value a setting
+/// already holds is not a change; changing a setting and changing it back is.
+/// </summary>
+public abstract class ConfigSection
+{
+    private bool _changed;
+
+    /// <summary>
+    /// Gets whether a setting of this section, or of a nested section, was changed.
+    /// </summary>
+    [JsonIgnore]
+    public virtual bool IsDirty => _changed;
+
+    /// <summary>
+    /// Clears the dirty flag of this section and its nested sections.
+    /// </summary>
+    public virtual void AcceptChanges() => _changed = false;
+
+    /// <summary>
+    /// Stores a setting and marks the section dirty when the value differs.
+    /// </summary>
+    /// <typeparam name="T">The setting type.</typeparam>
+    /// <param name="field">The backing field.</param>
+    /// <param name="value">The new value.</param>
+    protected void Set<T>(ref T field, T value)
+    {
+        if (!EqualityComparer<T>.Default.Equals(field, value))
+        {
+            field = value;
+            _changed = true;
+        }
+    }
+}
+
 /// <summary>
 /// Card ordering settings. The chosen order is global, not per workspace.
 /// </summary>
-public sealed class SortSettings
+public sealed class SortSettings : ConfigSection
 {
     /// <summary>
     /// Gets or sets the order used by the board.
     /// </summary>
     [JsonProperty("card")]
-    public CardSortOption Card { get; set; } = CardSortOption.Id;
+    public CardSortOption Card { get; set => Set(ref field, value); } = CardSortOption.Id;
 }
 
 /// <summary>
 /// Board column width settings, applied globally.
 /// </summary>
-public sealed class ColumnWidthSettings
+public sealed class ColumnWidthSettings : ConfigSection
 {
     /// <summary>
     /// Gets or sets the active width preset.
     /// </summary>
     [JsonProperty("preset")]
-    public ColumnWidthPreset Preset { get; set; } = ColumnWidthPreset.Standard;
+    public ColumnWidthPreset Preset { get; set => Set(ref field, value); } = ColumnWidthPreset.Standard;
 
     /// <summary>
     /// Gets or sets the manual width in pixels, used when <see cref="Preset"/>
     /// is <see cref="ColumnWidthPreset.Custom"/> (range 160–720).
     /// </summary>
     [JsonProperty("custom-pixels")]
-    public int CustomPixels { get; set; } = 320;
+    public int CustomPixels { get; set => Set(ref field, value); } = 320;
 }
 
 /// <summary>
 /// Confirmation-dialog toggles for the destructive actions.
 /// </summary>
-public sealed class ConfirmationSettings
+public sealed class ConfirmationSettings : ConfigSection
 {
     /// <summary>
     /// Gets or sets whether deleting a card asks for confirmation.
     /// </summary>
     [JsonProperty("delete-card")]
-    public bool DeleteCard { get; set; } = true;
+    public bool DeleteCard { get; set => Set(ref field, value); } = true;
 
     /// <summary>
     /// Gets or sets whether deleting a column asks for confirmation.
     /// </summary>
     [JsonProperty("delete-column")]
-    public bool DeleteColumn { get; set; } = true;
+    public bool DeleteColumn { get; set => Set(ref field, value); } = true;
 
     /// <summary>
     /// Gets or sets whether deleting a tag asks for confirmation.
     /// </summary>
     [JsonProperty("delete-tag")]
-    public bool DeleteTag { get; set; } = true;
+    public bool DeleteTag { get; set => Set(ref field, value); } = true;
 
     /// <summary>
     /// Gets or sets whether discarding card edits asks for confirmation.
     /// </summary>
     [JsonProperty("discard-edit")]
-    public bool DiscardEdit { get; set; } = true;
+    public bool DiscardEdit { get; set => Set(ref field, value); } = true;
 }
 
 /// <summary>
@@ -129,7 +165,7 @@ public sealed class ConfirmationSettings
 /// <see cref="JsonPropertyAttribute"/> and <see cref="EnumMemberAttribute"/>).
 /// The <c>version</c> field is the migration hook for future format changes.
 /// </summary>
-public sealed class AppConfig
+public sealed class AppConfig : ConfigSection
 {
     /// <summary>
     /// Current configuration format version.
@@ -165,35 +201,47 @@ public sealed class AppConfig
     /// Gets or sets the configuration format version of this file.
     /// </summary>
     [JsonProperty("version")]
-    public int Version { get; set; } = CurrentFormatVersion;
+    public int Version { get; set => Set(ref field, value); } = CurrentFormatVersion;
 
     /// <summary>
     /// Gets or sets the UI language code ("en" default or "zh-Hans").
     /// </summary>
     [JsonProperty("language")]
-    public string Language { get; set; } = DefaultLanguage;
+    public string Language { get; set => Set(ref field, value); } = DefaultLanguage;
 
     /// <summary>
     /// Gets or sets the UI theme selection (default: follow the system).
     /// </summary>
     [JsonProperty("theme")]
-    public ThemeOption Theme { get; set; } = ThemeOption.FollowSystem;
+    public ThemeOption Theme { get; set => Set(ref field, value); } = ThemeOption.FollowSystem;
 
     /// <summary>
     /// Gets or sets the card ordering settings.
     /// </summary>
     [JsonProperty("sort")]
-    public SortSettings Sort { get; set; } = new();
+    public SortSettings Sort { get; set => Set(ref field, value); } = new();
 
     /// <summary>
     /// Gets or sets the board column width settings.
     /// </summary>
     [JsonProperty("column-width")]
-    public ColumnWidthSettings ColumnWidth { get; set; } = new();
+    public ColumnWidthSettings ColumnWidth { get; set => Set(ref field, value); } = new();
 
     /// <summary>
     /// Gets or sets the confirmation-dialog settings.
     /// </summary>
     [JsonProperty("confirmations")]
-    public ConfirmationSettings Confirmations { get; set; } = new();
+    public ConfirmationSettings Confirmations { get; set => Set(ref field, value); } = new();
+
+    /// <inheritdoc/>
+    public override bool IsDirty => base.IsDirty || Sort.IsDirty || ColumnWidth.IsDirty || Confirmations.IsDirty;
+
+    /// <inheritdoc/>
+    public override void AcceptChanges()
+    {
+        base.AcceptChanges();
+        Sort.AcceptChanges();
+        ColumnWidth.AcceptChanges();
+        Confirmations.AcceptChanges();
+    }
 }
diff --git a/YKanBan/Storage/AppData/AppConfigStore.cs b/YKanBan/Storage/AppData/AppConfigStore.cs
index 66b0a2f..5cf8446 100644
--- a/YKanBan/Storage/AppData/AppConfigStore.cs
+++ b/YKanBan/Storage/AppData/AppConfigStore.cs
@@ -10,8 +10,8 @@ namespace YKanBan.Storage.AppData;
 /// then defaults are used; an individual invalid field (unknown enum value or
 /// language, out-of-range width, wrong type) falls back to its own default while
 /// the other fields are kept; a file that exists but cannot be read (I/O or
-/// permission error) also yields defaults. Saving happens only when the configuration differs
-/// from what was loaded, and writes the file directly (no atomic replace) —
+/// permission error) also yields defaults. Saving happens only when this session
+/// changed a setting (<see cref="ConfigSection.IsDirty"/>), and writes the file directly (no atomic replace) —
 /// losing the last session's settings to a crash is an accepted trade-off.
 /// </summary>
 public sealed class AppConfigStore
@@ -36,9 +36,6 @@ public sealed class AppConfigStore
         Error = (_, args) => args.ErrorContext.Handled = true,
     });
 
-    /// <summary>The serialized form of the configuration as last loaded or saved.</summary>
-    private string _persistedSnapshot = Serialize(new AppConfig());
-
     /// <summary>
     /// Initializes a store bound to a specific app.json path.
     /// </summary>
@@ -51,8 +48,8 @@ public sealed class AppConfigStore
     public string FilePath { get; }
 
     /// <summary>
-    /// Loads the configuration and records it as the persisted baseline for
-    /// <see cref="SaveIfChanged"/>. A missing file yields defaults; a file that
+    /// Loads the configuration with a clear dirty flag, the baseline for
+    /// <see cref="SaveIfDirty"/>. A missing file yields defaults; a file that
     /// is not a JSON object is backed up to app.json.bak (best effort) and then
     /// defaults are used; invalid fields fall back to their defaults
     /// individually; a file that cannot be read yields defaults. Never throws for
@@ -63,39 +60,37 @@ public sealed class AppConfigStore
     {
         AppConfig config = Read();
 
-        // The baseline is the effective configuration, so field fallbacks alone never trigger a write.
-        _persistedSnapshot = Serialize(config);
+        // Deserializing and field fallbacks go through the setters; neither is a change by this session.
+        config.AcceptChanges();
         return config;
     }
 
     /// <summary>
-    /// Writes the configuration only when it differs from the loaded or last
-    /// saved one, i.e. when this session changed a setting.
+    /// Writes the configuration only when a setting was changed since it was
+    /// loaded or last saved, even if it was later changed back.
     /// </summary>
     /// <param name="config">The live configuration.</param>
     /// <returns><see langword="true"/> when the file was written.</returns>
-    public bool SaveIfChanged(AppConfig config)
+    public bool SaveIfDirty(AppConfig config)
     {
-        string json = Serialize(config);
-        if (json == _persistedSnapshot)
+        if (!config.IsDirty)
         {
             return false;
         }
 
-        Write(json);
-        _persistedSnapshot = json;
+        Save(config);
         return true;
     }
 
     /// <summary>
-    /// Writes the configuration directly to disk (no atomic replace), unconditionally.
+    /// Writes the configuration directly to disk (no atomic replace),
+    /// unconditionally, and clears its dirty flag.
     /// </summary>
     /// <param name="config">The configuration to serialize.</param>
     public void Save(AppConfig config)
     {
-        string json = Serialize(config);
-        Write(json);
-        _persistedSnapshot = json;
+        Write(Serialize(config));
+        config.AcceptChanges();
     }
 
     /// <summary>
diff --git a/docs/ACCEPTANCE.md b/docs/ACCEPTANCE.md
index 153e717..e9868ed 100644
--- a/docs/ACCEPTANCE.md
+++ b/docs/ACCEPTANCE.md
@@ -41,6 +41,7 @@
 
 - [ ] 同时打开两个实例（不同工作区）。实例 1 改排序，实例 2 改列宽；先关实例 1、后关实例 2 → `app.json` 为实例 2 的完整设置（后退出者覆盖，实例 1 的排序改动丢失——已接受）
 - [ ] 打开两个实例，只在实例 1 改设置；先关实例 1，再关**未改过设置**的实例 2 → `app.json` 保持实例 1 写入的内容（未改设置的实例不写盘）
+- [ ] 删除 `app.json`，打开设置、不改任何项直接确认后退出 → `app.json` 不被创建；改一项再改回原值后退出 → `app.json` 被写出（脏标记按"改动过"判定）
 - [ ] 处于错误页（例如参数错误页）的实例退出时不写 `app.json`（可先删除 `app.json`，退出后确认文件未被创建）
 - [ ] `app.json` 写成非法 JSON → 启动正常、使用默认值，生成 `app.json.bak`；改一次设置退出后 `app.json` 恢复为合法内容
 - [ ] `app.json` 中单个字段非法（如 `"theme": "pink"`、列宽 `"custom-pixels": 9999`）→ 仅该字段回退默认，其余设置保留，不生成 `.bak`
diff --git a/docs/PLAN.md b/docs/PLAN.md
index b2839bd..49fb768 100644
--- a/docs/PLAN.md
+++ b/docs/PLAN.md
@@ -16,7 +16,7 @@
 - 网络共享明确不支持；命令行参数边界；macOS 无参启动行为写入文档（§2.3、§4.1、附录）
 - 补充：导出转义规则、卡片列表虚拟化、模态窗口 FlowDirection、`Resources` 类名使用规则、CI osx-x64 运行器（§5、§7、§8）
 - 依赖：Avalonia 11.3.6 → 11.3.22，Semi.Avalonia 11.2.1.10 → 11.3.22（§7）
-- 实现回补：搜索括号嵌套上限 64 层、单个表达式至多 256 个原子，空短语、短语紧贴文本、`key:` 后空白均为非法（§6.1、§6.2）；导出表格单元格转义 `\`、默认文件名剔除非法字符（§5.8）；初始化中途锁冲突进锁冲突页、数据库文件缺失不静默新建（§4.4）；app.json 读不出 / 备份写不出 / 退出写盘失败均不阻断启动与退出（§4.3）；主题下拉选中即预览、取消恢复（§5.6）；空白新卡片确认不创建（§5.4）
+- 实现回补：搜索括号嵌套上限 64 层、单个表达式至多 256 个原子，空短语、短语紧贴文本、`key:` 后空白均为非法（§6.1、§6.2）；导出表格单元格转义 `\`、默认文件名剔除非法字符（§5.8）；初始化中途锁冲突进锁冲突页、数据库文件缺失不静默新建（§4.4）；app.json 读不出 / 备份写不出 / 退出写盘失败均不阻断启动与退出（§4.3）；主题下拉选中即预览、取消恢复（§5.6）；空白新卡片确认不创建（§5.4）；app.json 脏标记按"改动过"判定（改回原值仍写盘）（§4.3）
 - 里程碑：标注 M0–M4 已完成，新增 **M4R（v1.9 回补）**（§9）
 
 ## 1. 项目概述
@@ -137,7 +137,7 @@ card_tags: (card_id, tag_id) 复合主键，双向 ON DELETE CASCADE
 
 - 强类型根对象 `AppConfig` 整体序列化，根含 `"version": 1` 迁移钩子
 - 字段：语言（`Language`：`"en"` 默认 / `"zh-Hans"`）、主题（`Theme`：`"follow-system"` 默认 / `"light"` / `"dark"`）、卡片排序选择（`"id"` 默认 / `"updated-at"` / `"title"`）、列宽、四类确认开关；枚举值序列化为 kebab-case 字符串
-- 写入时机：**应用退出时，仅当本实例在会话内修改过任一设置（脏标记）才整体写盘**（直接写，不做原子替换）；错误页状态的实例、未改过设置的实例退出时不写。多实例均改过设置时后退出者覆盖（已接受）；崩溃/断电丢失当次会话的设置变更（已接受：配置低价值）
+- 写入时机：**应用退出时，仅当本实例在会话内修改过任一设置（脏标记）才整体写盘**（直接写，不做原子替换）；脏标记按"是否改动过"判定：设置项被赋予与当前不同的值即置脏，改后又改回原值仍为脏；赋予相同值（如设置模态未改任何项直接确认）不置脏；读取时的字段回退不置脏；错误页状态的实例、未改过设置的实例退出时不写。多实例均改过设置时后退出者覆盖（已接受）；崩溃/断电丢失当次会话的设置变更（已接受：配置低价值）
 - 读取：
   - 文件缺失 → 默认值启动照常运行（下次脏退出时自然创建）
   - JSON 本身无法解析 → 备份为 `app.json.bak`（覆盖旧备份）→ 以默认值启动