diff --git a/YKanBan.Tests/Conventions/CodeBehindConventionTests.cs b/YKanBan.Tests/Conventions/CodeBehindConventionTests.cs
new file mode 100644
index 0000000..28390c0
--- /dev/null
+++ b/YKanBan.Tests/Conventions/CodeBehindConventionTests.cs
@@ -0,0 +1,60 @@
+using System.Runtime.CompilerServices;
+using System.Text.RegularExpressions;
+
+namespace YKanBan.Tests.Conventions;
+
+/// <summary>
+/// Source-level guards for code-behind conventions (plan §8.1): inside control,
+/// window and application code-behind a bare <c>Resources.Key</c> resolves to
+/// the <c>StyledElement.Resources</c> instance property, so the generated class
+/// must be written as <c>YKanBan.Resources</c> (or through a using alias).
+/// </summary>
+[TestClass]
+public partial class CodeBehindConventionTests
+{
+    /// <summary>A <c>Resources.</c> member access not preceded by a qualifier or identifier character.</summary>
+    [GeneratedRegex(@"(?<![\w.])Resources\.")]
+    private static partial Regex BareResourcesAccess();
+
+    /// <summary>
+    /// Locates the application project's source folder from this file's compile-time path.
+    /// </summary>
+    /// <param name="thisFile">Filled in by the compiler.</param>
+    /// <returns>The YKanBan project folder.</returns>
+    private static string ApplicationSourceFolder([CallerFilePath] string thisFile = "")
+    {
+        DirectoryInfo? directory = new FileInfo(thisFile).Directory;
+        while (directory is not null && !File.Exists(Path.Combine(directory.FullName, "YKanBan", "YKanBan.csproj")))
+        {
+            directory = directory.Parent;
+        }
+
+        Assert.IsNotNull(directory, "Repository root not found from the test source path.");
+        return Path.Combine(directory.FullName, "YKanBan");
+    }
+
+    [TestMethod]
+    public void CodeBehindNeverUsesBareResourcesClassName()
+    {
+        string root = ApplicationSourceFolder();
+        string[] files = Directory.GetFiles(root, "*.axaml.cs", SearchOption.AllDirectories)
+            .Where(path => !path.Split(Path.DirectorySeparatorChar).Any(part => part is "obj" or "bin"))
+            .ToArray();
+        Assert.IsTrue(files.Length > 0, "No code-behind files found.");
+
+        var offenders = new List<string>();
+        foreach (string file in files)
+        {
+            string[] lines = File.ReadAllLines(file);
+            for (int index = 0; index < lines.Length; index++)
+            {
+                if (BareResourcesAccess().IsMatch(lines[index]))
+                {
+                    offenders.Add($"{Path.GetRelativePath(root, file)}:{index + 1}: {lines[index].Trim()}");
+                }
+            }
+        }
+
+        Assert.AreEqual(0, offenders.Count, "Use YKanBan.Resources in code-behind:" + Environment.NewLine + string.Join(Environment.NewLine, offenders));
+    }
+}
diff --git a/YKanBan/App.axaml.cs b/YKanBan/App.axaml.cs
index 1977aec..dae177c 100644
--- a/YKanBan/App.axaml.cs
+++ b/YKanBan/App.axaml.cs
@@ -26,6 +26,12 @@ public partial class App : Application
     /// </summary>
     public static string StartupLanguage { get; private set; } = AppConfig.DefaultLanguage;
 
+    /// <summary>
+    /// Gets the flow direction of the startup culture. FlowDirection does not
+    /// inherit across windows, so the main window and every modal window apply it.
+    /// </summary>
+    public static FlowDirection UiFlowDirection { get; private set; } = FlowDirection.LeftToRight;
+
     public override void Initialize()
     {
         AvaloniaXamlLoader.Load(this);
@@ -43,19 +49,20 @@ public partial class App : Application
             CultureInfo culture = ResolveCulture(Services.Config.Language);
             StartupLanguage = Services.Config.Language;
             Resx.Culture = culture;
+
+            // RTL detection drives the flow direction of every window; the two
+            // supported languages are LTR, but the infrastructure stays in
+            // place for future languages.
+            UiFlowDirection = culture.TextInfo.IsRightToLeft
+                ? FlowDirection.RightToLeft
+                : FlowDirection.LeftToRight;
             ApplyTheme(Services.Config.Theme);
 
             var viewModel = new MainWindowViewModel(Services, desktop.Args ?? []);
             desktop.MainWindow = new MainWindow
             {
                 DataContext = viewModel,
-
-                // RTL detection drives the whole window flow direction; the two
-                // supported languages are LTR, but the infrastructure stays in
-                // place for future languages.
-                FlowDirection = culture.TextInfo.IsRightToLeft
-                    ? FlowDirection.RightToLeft
-                    : FlowDirection.LeftToRight,
+                FlowDirection = UiFlowDirection,
             };
         }
 
diff --git a/YKanBan/Views/MainWindow.axaml.cs b/YKanBan/Views/MainWindow.axaml.cs
index 6cbaf8f..0f19084 100644
--- a/YKanBan/Views/MainWindow.axaml.cs
+++ b/YKanBan/Views/MainWindow.axaml.cs
@@ -37,7 +37,7 @@ public partial class MainWindow : Window
         {
             DataContext = new SettingsDialogViewModel(App.Services.Config, App.StartupLanguage),
         };
-        await dialog.ShowDialog(this);
+        await ShowModalAsync(dialog);
     }
 
     /// <summary>Shows the about dialog centered on this window.</summary>
@@ -47,6 +47,18 @@ public partial class MainWindow : Window
         {
             DataContext = new AboutDialogViewModel(),
         };
-        await dialog.ShowDialog(this);
+        await ShowModalAsync(dialog);
+    }
+
+    /// <summary>
+    /// Shows a modal window owned by this window. Every modal goes through here:
+    /// FlowDirection does not inherit across windows, so it is applied explicitly.
+    /// </summary>
+    /// <param name="dialog">The modal window to show.</param>
+    /// <returns>A task completing when the dialog closes.</returns>
+    private Task ShowModalAsync(Window dialog)
+    {
+        dialog.FlowDirection = App.UiFlowDirection;
+        return dialog.ShowDialog(this);
     }
 }