diff --git a/src/OpenIPC.Viewer.App/Services/Localizer.cs b/src/OpenIPC.Viewer.App/Services/Localizer.cs
index 19ff488..b0c5403 100644
--- a/src/OpenIPC.Viewer.App/Services/Localizer.cs
+++ b/src/OpenIPC.Viewer.App/Services/Localizer.cs
@@ -255,6 +255,7 @@ private static LangCode DetectSystem()
 
         ["Settings.Advanced.VerboseLogging"] = "Verbose logging (debug level)",
         ["Settings.Advanced.RawConfigEditor"] = "Allow risky device tools (raw config editing, file manager)",
+        ["Settings.Advanced.SingleInstance"] = "Single instance only (a repeat launch focuses the running window)",
         ["Settings.Advanced.OpenAppData"] = "Open app data folder",
 
         ["Settings.About.Repository"] = "GitHub repository →",
@@ -728,6 +729,7 @@ private static LangCode DetectSystem()
 
         ["Settings.Advanced.VerboseLogging"] = "Подробные логи (debug)",
         ["Settings.Advanced.RawConfigEditor"] = "Разрешить рискованные инструменты (raw-конфиг, файловый менеджер)",
+        ["Settings.Advanced.SingleInstance"] = "Запрещать запуск второй копии (повторный запуск откроет это окно)",
         ["Settings.Advanced.OpenAppData"] = "Открыть папку данных",
 
         ["Settings.About.Repository"] = "Репозиторий на GitHub →",
diff --git a/src/OpenIPC.Viewer.App/Services/UserSettings.cs b/src/OpenIPC.Viewer.App/Services/UserSettings.cs
index 9320bf7..f0edcad 100644
--- a/src/OpenIPC.Viewer.App/Services/UserSettings.cs
+++ b/src/OpenIPC.Viewer.App/Services/UserSettings.cs
@@ -94,7 +94,11 @@ public sealed record UserSettings(
     int GridStillsIntervalSeconds = 10,
     // Desktop only: closing the main window hides it to the tray icon instead
     // of quitting (live streams are released while hidden). Off = close quits.
-    bool CloseToTray = false)
+    bool CloseToTray = false,
+    // Desktop only: refuse to start a second copy of the app — a repeat launch
+    // brings the already-running window to the foreground instead (see
+    // Desktop/SingleInstanceGuard). Off = any number of copies may run.
+    bool SingleInstance = false)
 {
     public static UserSettings Default => new();
 }
diff --git a/src/OpenIPC.Viewer.App/ViewModels/SettingsPageViewModel.cs b/src/OpenIPC.Viewer.App/ViewModels/SettingsPageViewModel.cs
index 2e2ec98..1589191 100644
--- a/src/OpenIPC.Viewer.App/ViewModels/SettingsPageViewModel.cs
+++ b/src/OpenIPC.Viewer.App/ViewModels/SettingsPageViewModel.cs
@@ -46,6 +46,7 @@ public sealed partial class SettingsPageViewModel : ViewModelBase
     [ObservableProperty] private string _language = "system";
     [ObservableProperty] private bool _showSplash = true;
     [ObservableProperty] private bool _closeToTray;
+    [ObservableProperty] private bool _singleInstance;
 
     // Gates the desktop-only toggles (tray) off the shared settings page.
     public bool IsDesktopPlatform { get; } =
@@ -216,6 +217,7 @@ private void Load()
             Language = s.Language;
             ShowSplash = s.ShowSplash;
             CloseToTray = s.CloseToTray;
+            SingleInstance = s.SingleInstance;
             SshStrictHostKey = s.SshStrictHostKey;
             SshDefaultPort = s.SshDefaultPort;
             SshTerminalFontSize = s.SshTerminalFontSize;
@@ -248,6 +250,7 @@ private void Load()
     partial void OnLanguageChanged(string value) => Persist();
     partial void OnShowSplashChanged(bool value) => Persist();
     partial void OnCloseToTrayChanged(bool value) => Persist();
+    partial void OnSingleInstanceChanged(bool value) => Persist();
     partial void OnSshStrictHostKeyChanged(bool value) => Persist();
     partial void OnSshDefaultPortChanged(int value) => Persist();
     partial void OnSshTerminalFontSizeChanged(int value) => Persist();
@@ -279,6 +282,7 @@ private void Persist()
             Language = Language,
             ShowSplash = ShowSplash,
             CloseToTray = CloseToTray,
+            SingleInstance = SingleInstance,
             SshStrictHostKey = SshStrictHostKey,
             SshDefaultPort = SshDefaultPort,
             SshTerminalFontSize = SshTerminalFontSize,
diff --git a/src/OpenIPC.Viewer.App/Views/Pages/SettingsPage.axaml b/src/OpenIPC.Viewer.App/Views/Pages/SettingsPage.axaml
index 6de5509..fea5c58 100644
--- a/src/OpenIPC.Viewer.App/Views/Pages/SettingsPage.axaml
+++ b/src/OpenIPC.Viewer.App/Views/Pages/SettingsPage.axaml
@@ -417,6 +417,11 @@
                     Content="{Binding [Settings.Advanced.RawConfigEditor], Source={x:Static svc:Localizer.Instance}}"
                     Foreground="{StaticResource TextPrimaryBrush}" />
 
+          <CheckBox IsChecked="{Binding SingleInstance}"
+                    IsVisible="{Binding IsDesktopPlatform}"
+                    Content="{Binding [Settings.Advanced.SingleInstance], Source={x:Static svc:Localizer.Instance}}"
+                    Foreground="{StaticResource TextPrimaryBrush}" />
+
           <StackPanel Spacing="6">
             <Button Classes="outlined"
                     Content="{Binding [Settings.Advanced.OpenAppData], Source={x:Static svc:Localizer.Instance}}"
diff --git a/src/OpenIPC.Viewer.App/Views/Pages/SettingsPage.axaml.cs b/src/OpenIPC.Viewer.App/Views/Pages/SettingsPage.axaml.cs
index c1ef507..aff1a1e 100644
--- a/src/OpenIPC.Viewer.App/Views/Pages/SettingsPage.axaml.cs
+++ b/src/OpenIPC.Viewer.App/Views/Pages/SettingsPage.axaml.cs
@@ -2,6 +2,7 @@
 using System.ComponentModel;
 using Avalonia;
 using Avalonia.Controls;
+using Avalonia.Threading;
 using OpenIPC.Viewer.App.ViewModels;
 
 namespace OpenIPC.Viewer.App.Views.Pages;
@@ -14,6 +15,7 @@ public sealed partial class SettingsPage : UserControl
     private const double WideThreshold = 700;
 
     private SettingsPageViewModel? _vm;
+    private bool _relayoutQueued;
 
     public SettingsPage()
     {
@@ -57,6 +59,20 @@ private void OnSizeChanged(object? sender, SizeChangedEventArgs e)
             _vm.IsWide = nextWide;  // triggers ShowList/ShowDetail change → ApplyLayout
         else
             ApplyLayout();
+
+        // During an interactive resize Avalonia can render with a stale
+        // arrange pass and (rarely, seen on Windows) miss the final one,
+        // leaving the detail pane clipped at the window edge. Queue a single
+        // low-priority re-apply + re-measure so a definitive layout pass runs
+        // after the resize storm settles. Coalesced: one pending post at most.
+        if (_relayoutQueued) return;
+        _relayoutQueued = true;
+        Dispatcher.UIThread.Post(() =>
+        {
+            _relayoutQueued = false;
+            ApplyLayout();
+            RootGrid.InvalidateMeasure();
+        }, DispatcherPriority.Background);
     }
 
     private void ApplyLayout()
diff --git a/src/OpenIPC.Viewer.Desktop/Program.cs b/src/OpenIPC.Viewer.Desktop/Program.cs
index acbeb98..41cc454 100644
--- a/src/OpenIPC.Viewer.Desktop/Program.cs
+++ b/src/OpenIPC.Viewer.Desktop/Program.cs
@@ -16,6 +16,15 @@ public static int Main(string[] args)
         System.Diagnostics.Trace.Listeners.Add(new System.Diagnostics.ConsoleTraceListener());
 #endif
 
+        // Opt-in single-instance mode: when another copy already runs and the
+        // user enabled the guard, hand focus to it and bail out before any
+        // services (logs, db) spin up.
+        if (!SingleInstanceGuard.TryBecomePrimary() && SingleInstanceGuard.IsEnabledInSettings())
+        {
+            SingleInstanceGuard.ActivatePrimary();
+            return 0;
+        }
+
         var services = Composition.Build();
         App.App.Services = services;
 
diff --git a/src/OpenIPC.Viewer.Desktop/SingleInstanceGuard.cs b/src/OpenIPC.Viewer.Desktop/SingleInstanceGuard.cs
new file mode 100644
index 0000000..997fdc8
--- /dev/null
+++ b/src/OpenIPC.Viewer.Desktop/SingleInstanceGuard.cs
@@ -0,0 +1,118 @@
+using System;
+using System.IO;
+using System.IO.Pipes;
+using System.Text.Json;
+using System.Threading;
+using Avalonia.Controls;
+using Avalonia.Controls.ApplicationLifetimes;
+using Avalonia.Threading;
+using OpenIPC.Viewer.App;
+
+namespace OpenIPC.Viewer.Desktop;
+
+// Opt-in single-instance mode (Settings → Advanced). The first process always
+// takes the named mutex and serves the activation pipe, even with the setting
+// off — that way enabling the toggle later protects against copies launched
+// afterwards without restarting the app that's already running. A losing
+// process consults the setting itself (read straight from usersettings.json,
+// before DI or Avalonia spin up) and, when enforcement is on, pings the pipe
+// so the existing window comes to the foreground, then exits.
+internal static class SingleInstanceGuard
+{
+    // User-scoped names so two OS sessions each get their own slot. "Local\"
+    // confines the mutex to the current session on Windows; other platforms
+    // treat the prefix as part of the name, which is equally fine.
+    private static readonly string MutexName =
+        @"Local\OpenIPC.Viewer.instance." + Uri.EscapeDataString(Environment.UserName);
+    private static readonly string PipeName =
+        "OpenIPC.Viewer.activate." + Uri.EscapeDataString(Environment.UserName);
+
+    private static Mutex? _mutex; // held for the process lifetime on purpose
+
+    // True → this process is the primary: it owns the mutex and answers
+    // activation pings. False → another copy already runs.
+    public static bool TryBecomePrimary()
+    {
+        var mutex = new Mutex(initiallyOwned: true, MutexName, out var createdNew);
+        if (!createdNew)
+        {
+            mutex.Dispose();
+            return false;
+        }
+        _mutex = mutex;
+        StartActivationListener();
+        return true;
+    }
+
+    // The user's toggle, read without the service stack: Main needs the answer
+    // before Composition.Build() opens logs/db. Property casing matches what
+    // UserSettingsService writes (PascalCase, no naming policy).
+    public static bool IsEnabledInSettings()
+    {
+        try
+        {
+            var path = Path.Combine(AppPaths.AppDataDir.FullName, "usersettings.json");
+            if (!File.Exists(path)) return false;
+            using var doc = JsonDocument.Parse(File.ReadAllBytes(path));
+            return doc.RootElement.TryGetProperty("SingleInstance", out var v)
+                   && v.ValueKind == JsonValueKind.True;
+        }
+        catch
+        {
+            return false; // corrupt/unreadable settings must never block startup
+        }
+    }
+
+    // Second-instance side: ask the primary to come to the foreground.
+    // Best-effort — if the primary is mid-startup or wedged, we just exit.
+    public static void ActivatePrimary()
+    {
+        try
+        {
+            using var client = new NamedPipeClientStream(".", PipeName, PipeDirection.Out);
+            client.Connect(timeout: 2000);
+            client.WriteByte(1);
+            client.Flush();
+        }
+        catch
+        {
+            // Nothing useful to do: the user still sees the primary window
+            // (possibly minimized), and this process is about to exit anyway.
+        }
+    }
+
+    private static void StartActivationListener()
+    {
+        var thread = new Thread(() =>
+        {
+            while (true)
+            {
+                try
+                {
+                    using var server = new NamedPipeServerStream(PipeName, PipeDirection.In, maxNumberOfServerInstances: 1);
+                    server.WaitForConnection();
+                    server.ReadByte(); // any payload = "bring yourself to front"
+                    Dispatcher.UIThread.Post(ActivateMainWindow);
+                }
+                catch
+                {
+                    // Client vanished mid-handshake or the dispatcher isn't up
+                    // yet — drop this ping and keep serving the next one.
+                }
+            }
+        })
+        { IsBackground = true, Name = "single-instance-activate" };
+        thread.Start();
+    }
+
+    private static void ActivateMainWindow()
+    {
+        if (Avalonia.Application.Current?.ApplicationLifetime
+            is not IClassicDesktopStyleApplicationLifetime { MainWindow: { } window })
+            return;
+        window.Show(); // un-hides a close-to-tray window
+        if (window.WindowState == WindowState.Minimized)
+            window.WindowState = WindowState.Normal;
+        window.Activate();
+    }
+}