changed.更了好多

2026-06-20 23:54:26 +08:00 · 2026-05-12 16:46:49 +08:00
parent 563f12caa1
commit 33c264f6dd
127 changed files with 5257 additions and 10534 deletions
--- a/LanMountainDesktop.Launcher/App.axaml
+++ b/LanMountainDesktop.Launcher/App.axaml
@@ -5,5 +5,8 @@
             RequestedThemeVariant="Default">
  <Application.Styles>
    <sty:FluentAvaloniaTheme />
+    <Style Selector="Window">
+      <Setter Property="Topmost" Value="True" />
+    </Style>
  </Application.Styles>
 </Application>
--- a/LanMountainDesktop.Launcher/App.axaml.cs
+++ b/LanMountainDesktop.Launcher/App.axaml.cs
@@ -112,6 +112,15 @@ public partial class App : Application
                _ = WaitForWindowCloseAsync(desktop, errorWindow);
                return true;
            }
+            case "preview-multi-instance":
+            {
+                Logger.Info("Preview command: multi-instance prompt.");
+                var promptWindow = new MultiInstancePromptWindow();
+                promptWindow.SetDetails(Environment.ProcessId, "ForegroundDesktop");
+                promptWindow.Show();
+                _ = WaitForWindowCloseAsync(desktop, promptWindow);
+                return true;
+            }
            case "preview-update":
            {
                Logger.Info("Preview command: update.");
--- a/LanMountainDesktop.Launcher/Services/HostAppSettingsOobeMerger.cs
+++ b/LanMountainDesktop.Launcher/Services/HostAppSettingsOobeMerger.cs
@@ -16,6 +16,7 @@ public static class HostAppSettingsOobeMerger
    public const string EnableFusedDesktopKey = "EnableFusedDesktop";
    public const string EnableThreeFingerSwipeKey = "EnableThreeFingerSwipe";
    public const string AutoStartWithWindowsKey = "AutoStartWithWindows";
+    public const string MultiInstanceLaunchBehaviorKey = "MultiInstanceLaunchBehavior";

    public static string GetSettingsFilePath(string dataRoot) =>
        Path.Combine(Path.GetFullPath(dataRoot.TrimEnd(Path.DirectorySeparatorChar, Path.AltDirectorySeparatorChar)), "settings.json");
@@ -54,6 +55,30 @@ public static class HostAppSettingsOobeMerger
        }
    }

+    public static MultiInstanceLaunchBehavior LoadMultiInstanceLaunchBehavior(string settingsPath)
+    {
+        if (!File.Exists(settingsPath))
+        {
+            return MultiInstanceLaunchBehavior.NotifyAndOpenDesktop;
+        }
+
+        try
+        {
+            var root = JsonNode.Parse(File.ReadAllText(settingsPath))?.AsObject();
+            if (root is null)
+            {
+                return MultiInstanceLaunchBehavior.NotifyAndOpenDesktop;
+            }
+
+            return ReadMultiInstanceLaunchBehavior(root);
+        }
+        catch (Exception ex)
+        {
+            Logger.Warn($"HostAppSettingsOobeMerger: failed to read multi-instance behavior from '{settingsPath}'. {ex.Message}");
+            return MultiInstanceLaunchBehavior.NotifyAndOpenDesktop;
+        }
+    }
+
    public static void MergeStartupPresentation(string settingsPath, HostAppSettingsStartupChoices choices)
    {
        var directory = Path.GetDirectoryName(settingsPath);
@@ -109,6 +134,31 @@ public static class HostAppSettingsOobeMerger
            _ => defaultValue
        };
    }
+
+    private static MultiInstanceLaunchBehavior ReadMultiInstanceLaunchBehavior(JsonObject root)
+    {
+        if (!root.TryGetPropertyValue(MultiInstanceLaunchBehaviorKey, out var node) || node is null)
+        {
+            return MultiInstanceLaunchBehavior.NotifyAndOpenDesktop;
+        }
+
+        if (node is JsonValue value)
+        {
+            if (value.TryGetValue<string>(out var text) &&
+                Enum.TryParse<MultiInstanceLaunchBehavior>(text, ignoreCase: true, out var parsed))
+            {
+                return parsed;
+            }
+
+            if (value.TryGetValue<int>(out var numeric) &&
+                Enum.IsDefined(typeof(MultiInstanceLaunchBehavior), numeric))
+            {
+                return (MultiInstanceLaunchBehavior)numeric;
+            }
+        }
+
+        return MultiInstanceLaunchBehavior.NotifyAndOpenDesktop;
+    }
 }

 public readonly record struct HostAppSettingsStartupDefaults(
--- a/LanMountainDesktop.Launcher/Services/Ipc/LauncherIpcServer.cs
+++ b/LanMountainDesktop.Launcher/Services/Ipc/LauncherIpcServer.cs
@@ -1,210 +0,0 @@
-using System.Buffers;
-using System.IO.Pipes;
-using System.Text.Json;
-using LanMountainDesktop.Shared.Contracts.Launcher;
-
-namespace LanMountainDesktop.Launcher.Services.Ipc;
-
-/// <summary>
-/// Launcher IPC 服务端 - 接收主程序的启动进度报告
-/// 采用持久连接 + 长度前缀协议，支持客户端在同一连接上多次发送消息。
-/// 跨平台实现：Windows 使用命名管道，Linux/macOS 使用 Unix 域套接字
-/// </summary>
-public class LauncherIpcServer : IDisposable
-{
-    private readonly CancellationTokenSource _cts = new();
-    private readonly Action<StartupProgressMessage> _onProgress;
-    private Task? _listenTask;
-    private NamedPipeServerStream? _currentPipe;
-
-    /// <summary>
-    /// 协议：每条消息以 4 字节小端 int32 长度前缀开头，后跟 UTF-8 JSON 正文。
-    /// 这在 Windows Message 模式和 unix Byte 模式下均能可靠工作。
-    /// </summary>
-    private const int LengthPrefixSize = 4;
-
-    private const int BackoffBaseMs = 200;
-    private const int BackoffMaxMs = 5000;
-    private const int BackoffJitterMs = 100;
-
-    public LauncherIpcServer(Action<StartupProgressMessage> onProgress)
-    {
-        _onProgress = onProgress;
-    }
-
-    /// <summary>
-    /// 启动 IPC 服务端监听
-    /// </summary>
-    public void Start()
-    {
-        _listenTask = Task.Run(ListenLoopAsync, _cts.Token);
-    }
-
-    private async Task ListenLoopAsync()
-    {
-        var consecutiveErrors = 0;
-
-        while (!_cts.Token.IsCancellationRequested)
-        {
-            NamedPipeServerStream? pipe = null;
-            try
-            {
-                pipe = new NamedPipeServerStream(
-                    LauncherIpcConstants.PipeName,
-                    PipeDirection.In,
-                    1,
-                    PipeTransmissionMode.Byte,
-                    PipeOptions.Asynchronous);
-
-                _currentPipe = pipe;
-                await pipe.WaitForConnectionAsync(_cts.Token);
-
-                consecutiveErrors = 0;
-
-                await ReadMessagesFromConnectionAsync(pipe, _cts.Token);
-            }
-            catch (OperationCanceledException)
-            {
-                break;
-            }
-            catch (IOException)
-            {
-                consecutiveErrors = 0;
-                continue;
-            }
-            catch (ObjectDisposedException)
-            {
-                break;
-            }
-            catch (Exception ex)
-            {
-                consecutiveErrors++;
-                var delay = ComputeBackoff(consecutiveErrors);
-                Console.Error.WriteLine($"IPC listen error (attempt {consecutiveErrors}), retrying in {delay}ms: {ex.Message}");
-                try
-                {
-                    await Task.Delay(delay, _cts.Token);
-                }
-                catch (OperationCanceledException)
-                {
-                    break;
-                }
-            }
-            finally
-            {
-                try
-                {
-                    pipe?.Dispose();
-                }
-                catch { }
-
-                if (ReferenceEquals(_currentPipe, pipe))
-                {
-                    _currentPipe = null;
-                }
-            }
-        }
-    }
-
-    private int ComputeBackoff(int attempt)
-    {
-        var exponential = BackoffBaseMs * (1 << Math.Min(attempt - 1, 5));
-        var capped = Math.Min(exponential, BackoffMaxMs);
-        var jitter = Random.Shared.Next(0, BackoffJitterMs);
-        return capped + jitter;
-    }
-
-    /// <summary>
-    /// 从已连接的管道中持续读取消息，直到连接断开或取消
-    /// </summary>
-    private async Task ReadMessagesFromConnectionAsync(NamedPipeServerStream pipe, CancellationToken cancellationToken)
-    {
-        var lengthBuffer = ArrayPool<byte>.Shared.Rent(LengthPrefixSize);
-        try
-        {
-            while (pipe.IsConnected && !cancellationToken.IsCancellationRequested)
-            {
-                // 1. 读取 4 字节长度前缀
-                var totalRead = 0;
-                while (totalRead < LengthPrefixSize)
-                {
-                    var read = await pipe.ReadAsync(lengthBuffer.AsMemory(totalRead, LengthPrefixSize - totalRead), cancellationToken);
-                    if (read == 0)
-                    {
-                        // 连接已关闭
-                        return;
-                    }
-                    totalRead += read;
-                }
-
-                var payloadLength = BitConverter.ToInt32(lengthBuffer, 0);
-                if (payloadLength <= 0 || payloadLength > 1024 * 1024) // 最大 1MB 单条消息
-                {
-                    // 无效长度，跳过此连接
-                    return;
-                }
-
-                // 2. 读取消息正文
-                var payloadBuffer = ArrayPool<byte>.Shared.Rent(payloadLength);
-                try
-                {
-                    totalRead = 0;
-                    while (totalRead < payloadLength)
-                    {
-                        var read = await pipe.ReadAsync(payloadBuffer.AsMemory(totalRead, payloadLength - totalRead), cancellationToken);
-                        if (read == 0)
-                        {
-                            return;
-                        }
-                        totalRead += read;
-                    }
-
-                    // 3. 反序列化并回调
-                    var json = System.Text.Encoding.UTF8.GetString(payloadBuffer, 0, payloadLength);
-                    var message = JsonSerializer.Deserialize(json, AppJsonContext.Default.StartupProgressMessage);
-                    if (message is not null)
-                    {
-                        _onProgress(message);
-                    }
-                }
-                catch (JsonException)
-                {
-                    // 忽略解析错误，继续读取下一条消息
-                }
-                finally
-                {
-                    ArrayPool<byte>.Shared.Return(payloadBuffer);
-                }
-            }
-        }
-        finally
-        {
-            ArrayPool<byte>.Shared.Return(lengthBuffer);
-        }
-    }
-
-    /// <summary>
-    /// 停止 IPC 服务端
-    /// </summary>
-    public void Stop()
-    {
-        _cts.Cancel();
-        try
-        {
-            _currentPipe?.Dispose();
-        }
-        catch { }
-    }
-
-    public void Dispose()
-    {
-        Stop();
-        _cts.Dispose();
-
-        try
-        {
-            _listenTask?.Wait(TimeSpan.FromSeconds(2));
-        }
-        catch { }
-    }
-}
--- a/LanMountainDesktop.Launcher/Services/LauncherFlowCoordinator.cs
+++ b/LanMountainDesktop.Launcher/Services/LauncherFlowCoordinator.cs
@@ -218,56 +218,53 @@ internal sealed class LauncherFlowCoordinator
            {
                if (ShouldProbeExistingHostBeforeLaunch(_context))
                {
-                    var existingActivation = await TryActivateExistingHostWithStatusAsync(ipcClient, TimeSpan.FromMilliseconds(900))
+                    var multiInstanceBehavior = LoadMultiInstanceLaunchBehavior();
+                    var existingShellStatus = await TryGetExistingHostStatusAsync(ipcClient, TimeSpan.FromMilliseconds(900))
                        .ConfigureAwait(false);
-                    if (existingActivation is not null)
+                    if (IsExistingHostReadyForLauncherDecision(existingShellStatus))
                    {
                        ipcConnected = true;
-                        shellStatus = existingActivation.Status;
-                        var recoverableActivationFailure = IsRecoverableActivationFailure(existingActivation);
-                        lastStage = existingActivation.Accepted
+                        shellStatus = existingShellStatus;
+                        var decisionResult = await ApplyExistingHostBehaviorAsync(
+                                ipcClient,
+                                multiInstanceBehavior,
+                                existingShellStatus!)
+                            .ConfigureAwait(false);
+                        shellStatus = decisionResult.ActivationResult?.Status ?? existingShellStatus;
+                        var recoverableActivationFailure = decisionResult.ActivationResult is not null &&
+                                                           IsRecoverableActivationFailure(decisionResult.ActivationResult);
+                        lastStage = decisionResult.Success || recoverableActivationFailure
                            ? StartupStage.ActivationRedirected
                            : StartupStage.ActivationFailed;
-                        lastStageMessage = existingActivation.Message;
-                        if (existingActivation.Accepted)
+                        lastStageMessage = decisionResult.Message;
+                        if (decisionResult.Success || recoverableActivationFailure)
                        {
                            _startupAttemptRegistry.MarkOwnedSucceeded(lastStage, lastStageMessage);
                        }
-                        else if (recoverableActivationFailure)
-                        {
-                            _startupAttemptRegistry.MarkOwnedWaitingForShell(lastStageMessage);
-                        }
                        else
                        {
                            _startupAttemptRegistry.MarkOwnedFailed(lastStage, lastStageMessage);
                        }

-                        PublishCoordinatorStatus(
-                            hostProcessAliveOverride: true,
-                            completed: true,
-                            succeeded: existingActivation.Accepted || recoverableActivationFailure);
+                        PublishCoordinatorStatus(hostProcessAliveOverride: true, completed: true, succeeded: decisionResult.Success);
                        windowsClosingByCoordinator = true;
                        await CloseWindowsAsync(splashWindow, loadingDetailsWindow).ConfigureAwait(false);
                        return BuildResult(
-                            success: existingActivation.Accepted || recoverableActivationFailure,
+                            success: decisionResult.Success,
                            stage: "launch",
-                            code: existingActivation.Accepted
-                                ? "existing_host_activated"
-                                : recoverableActivationFailure
-                                    ? "existing_host_startup_pending"
-                                    : "existing_host_activation_failed",
-                            message: recoverableActivationFailure
-                                ? "Existing desktop process is still starting; Launcher will not start another process."
-                                : existingActivation.Message,
+                            code: decisionResult.Code,
+                            message: decisionResult.Message,
                            details: MergeDetails(
                                launcherContextDetails,
                                new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase)
                                {
                                    ["publicIpcConnected"] = "true",
-                                    ["existingHostPid"] = existingActivation.Status.ProcessId.ToString(),
-                                    ["existingShellState"] = existingActivation.Status.ShellState,
-                                    ["existingTrayState"] = existingActivation.Status.Tray.State,
-                                    ["existingTaskbarUsable"] = existingActivation.Status.Taskbar.IsUsable.ToString()
+                                    ["multiInstanceBehavior"] = multiInstanceBehavior.ToString(),
+                                    ["existingHostPid"] = shellStatus?.ProcessId.ToString() ?? string.Empty,
+                                    ["existingShellState"] = shellStatus?.ShellState ?? string.Empty,
+                                    ["existingTrayState"] = shellStatus?.Tray.State ?? string.Empty,
+                                    ["existingTaskbarUsable"] = shellStatus?.Taskbar.IsUsable.ToString() ?? string.Empty,
+                                    ["activationAccepted"] = decisionResult.ActivationResult?.Accepted.ToString() ?? string.Empty
                                }));
                    }
                }
@@ -492,7 +489,7 @@ internal sealed class LauncherFlowCoordinator
                var connected = await TryConnectToPublicIpcAsync(ipcClient, TimeSpan.FromMilliseconds(1200)).ConfigureAwait(false);
                if (!connected)
                {
-                    Logger.Warn("Timed out waiting for host public IPC. Launcher will continue without live startup notifications.");
+                    Logger.Info("Host public IPC is not ready yet. Launcher will keep monitoring the host process and retry.");
                }
                else
                {
@@ -557,30 +554,7 @@ internal sealed class LauncherFlowCoordinator
                                    recoveryActivationAttempted: true));
                        }

-                        var retryOutcome = await RetryActivationAfterEarlyFailureAsync().ConfigureAwait(false);
-                        if (retryOutcome is not null)
-                        {
-                            windowsClosingByCoordinator = true;
-                            if (retryOutcome.Success)
-                            {
-                                _startupAttemptRegistry.MarkOwnedSucceeded(lastStage, retryOutcome.Message);
-                                PublishCoordinatorStatus(
-                                    hostProcessAliveOverride: !launchOutcome.Process.HasExited,
-                                    completed: true,
-                                    succeeded: true);
-                            }
-                            else
-                            {
-                                _startupAttemptRegistry.MarkOwnedFailed(lastStage, activationFailureReason);
-                                PublishCoordinatorStatus(
-                                    hostProcessAliveOverride: !launchOutcome.Process.HasExited,
-                                    completed: true,
-                                    succeeded: false);
-                            }
-
-                            await CloseWindowsAsync(splashWindow, loadingDetailsWindow).ConfigureAwait(false);
-                            return WithAdditionalDetails(retryOutcome, ComposeLaunchDetails(!launchOutcome.Process.HasExited, recoveryActivationAttempted: true));
-                        }
+                        Logger.Info("Activation failure did not recover through public IPC yet. Launcher will keep monitoring the current host attempt.");
                    }

                    if (processExitTask.IsCompleted)
@@ -589,7 +563,7 @@ internal sealed class LauncherFlowCoordinator
                        Logger.Warn($"Host exited before startup success criteria were met. ExitCode={exitCode}.");

                        windowsClosingByCoordinator = true;
-                        if (exitCode == HostExitCodes.SecondaryActivationSucceeded)
+                        if (IsSuccessfulActivationExitCode(exitCode))
                        {
                            _startupAttemptRegistry.MarkOwnedSucceeded(StartupStage.ActivationRedirected, "Host redirected activation to the existing desktop instance.");
                            PublishCoordinatorStatus(hostProcessAliveOverride: false, completed: true, succeeded: true);
@@ -608,7 +582,7 @@ internal sealed class LauncherFlowCoordinator
                        }

                        if (!activationRetryAttempted &&
-                            exitCode is HostExitCodes.SecondaryActivationFailed or HostExitCodes.RestartLockNotAcquired)
+                            IsFailedActivationExitCode(exitCode))
                        {
                            activationRetryAttempted = true;
                            var activationRecovery = await TryRecoverActivationThroughExistingHostAsync(
@@ -633,30 +607,7 @@ internal sealed class LauncherFlowCoordinator
                                        }));
                            }

-                            var retryOutcome = await RetryActivationAfterEarlyFailureAsync().ConfigureAwait(false);
-                            if (retryOutcome is not null)
-                            {
-                                if (retryOutcome.Success)
-                                {
-                                    _startupAttemptRegistry.MarkOwnedSucceeded(lastStage, retryOutcome.Message);
-                                    PublishCoordinatorStatus(hostProcessAliveOverride: false, completed: true, succeeded: true);
-                                }
-                                else
-                                {
-                                    _startupAttemptRegistry.MarkOwnedFailed(lastStage, activationFailureReason);
-                                    PublishCoordinatorStatus(hostProcessAliveOverride: false, completed: true, succeeded: false);
-                                }
-
-                                await CloseWindowsAsync(splashWindow, loadingDetailsWindow).ConfigureAwait(false);
-                                return WithAdditionalDetails(
-                                    retryOutcome,
-                                    MergeDetails(
-                                        ComposeLaunchDetails(hostProcessAlive: false, recoveryActivationAttempted: true),
-                                        new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase)
-                                        {
-                                            ["exitCode"] = exitCode.ToString()
-                                        }));
-                            }
+                            Logger.Info("Activation exit code did not recover through public IPC. Launcher will report the activation failure without launching another host.");
                        }

                        _startupAttemptRegistry.MarkOwnedFailed(lastStage, activationFailureReason);
@@ -665,10 +616,10 @@ internal sealed class LauncherFlowCoordinator
                        return BuildResult(
                            success: false,
                            stage: "launch",
-                            code: exitCode is HostExitCodes.SecondaryActivationFailed or HostExitCodes.RestartLockNotAcquired
+                            code: IsFailedActivationExitCode(exitCode)
                                ? "activation_failed"
                                : "host_exited_early",
-                            message: exitCode is HostExitCodes.SecondaryActivationFailed or HostExitCodes.RestartLockNotAcquired
+                            message: IsFailedActivationExitCode(exitCode)
                                ? $"Host activation handshake failed before the required startup state was reported. ExitCode={exitCode}."
                                : $"Host exited before the required startup state was reported. ExitCode={exitCode}.",
                            details: MergeDetails(
@@ -909,54 +860,6 @@ internal sealed class LauncherFlowCoordinator
        }
    }

-    private async Task<LauncherResult?> RetryActivationAfterEarlyFailureAsync()
-    {
-        Logger.Warn("Attempting one explicit activation retry after host early failure.");
-        var retryOutcome = await LaunchHostWithIpcAsync(forceDirectMode: true, retryTag: "explicit-activation-retry").ConfigureAwait(false);
-        if (!retryOutcome.Result.Success)
-        {
-            return retryOutcome.Result;
-        }
-
-        if (retryOutcome.ImmediateResult is not null)
-        {
-            return retryOutcome.ImmediateResult;
-        }
-
-        if (retryOutcome.Process is not null)
-        {
-            var retryExitTask = retryOutcome.Process.WaitForExitAsync();
-            var completed = await Task.WhenAny(retryExitTask, Task.Delay(TimeSpan.FromSeconds(15))).ConfigureAwait(false);
-
-            if (completed != retryExitTask)
-            {
-                return BuildResult(
-                    success: true,
-                    stage: "launch",
-                    code: "activation_retry_started",
-                    message: "Activation retry started the host successfully.",
-                    details: retryOutcome.Details);
-            }
-
-            if (retryOutcome.Process.ExitCode == HostExitCodes.SecondaryActivationSucceeded)
-            {
-                return BuildResult(
-                    success: true,
-                    stage: "launch",
-                    code: "activation_redirected",
-                    message: "Activation retry redirected to the existing desktop instance.",
-                    details: retryOutcome.Details);
-            }
-        }
-
-        return BuildResult(
-            success: false,
-            stage: "launch",
-            code: "activation_failed",
-            message: "Activation retry failed to make the desktop visible.",
-            details: retryOutcome.Details);
-    }
-
    private static async Task CloseWindowsAsync(SplashWindow splashWindow, LoadingDetailsWindow? loadingDetailsWindow)
    {
        try
@@ -1087,7 +990,7 @@ internal sealed class LauncherFlowCoordinator
            previousAttempt is null ? null : finalAttempt,
            !finalAttempt.ProcessCreated
                ? "start"
-                : finalAttempt.ExitCode is HostExitCodes.SecondaryActivationFailed or HostExitCodes.RestartLockNotAcquired
+                : finalAttempt.ExitCode is int finalExitCode && IsFailedActivationExitCode(finalExitCode)
                    ? "activation"
                    : "early-exit");

@@ -1101,7 +1004,7 @@ internal sealed class LauncherFlowCoordinator
                details));
        }

-        if (finalAttempt.ExitCode == HostExitCodes.SecondaryActivationSucceeded)
+        if (finalAttempt.ExitCode is not null && IsSuccessfulActivationExitCode(finalAttempt.ExitCode.Value))
        {
            return HostLaunchOutcome.FromImmediateResult(BuildResult(
                true,
@@ -1111,7 +1014,7 @@ internal sealed class LauncherFlowCoordinator
                details));
        }

-        if (finalAttempt.ExitCode is HostExitCodes.SecondaryActivationFailed or HostExitCodes.RestartLockNotAcquired)
+        if (finalAttempt.ExitCode is not null && IsFailedActivationExitCode(finalAttempt.ExitCode.Value))
        {
            return HostLaunchOutcome.FromResult(BuildResult(
                false,
@@ -1469,12 +1372,12 @@ internal sealed class LauncherFlowCoordinator
        }
        catch (Exception ex)
        {
-            Logger.Warn($"Public IPC connect failed: {ex.Message}");
+            Logger.Info($"Public IPC is not ready yet: {ex.Message}");
            return false;
        }
    }

-    private static bool ShouldProbeExistingHostBeforeLaunch(CommandContext context)
+    internal static bool ShouldProbeExistingHostBeforeLaunch(CommandContext context)
    {
        if (!string.Equals(context.Command, "launch", StringComparison.OrdinalIgnoreCase))
        {
@@ -1489,6 +1392,169 @@ internal sealed class LauncherFlowCoordinator
        return !string.Equals(context.LaunchSource, "restart", StringComparison.OrdinalIgnoreCase);
    }

+    private MultiInstanceLaunchBehavior LoadMultiInstanceLaunchBehavior()
+    {
+        try
+        {
+            var settingsPath = HostAppSettingsOobeMerger.GetSettingsFilePath(_dataLocationResolver.ResolveDataRoot());
+            return HostAppSettingsOobeMerger.LoadMultiInstanceLaunchBehavior(settingsPath);
+        }
+        catch (Exception ex)
+        {
+            Logger.Warn($"Failed to load multi-instance launch behavior. Falling back to default. {ex.Message}");
+            return MultiInstanceLaunchBehavior.NotifyAndOpenDesktop;
+        }
+    }
+
+    internal static bool IsExistingHostReadyForLauncherDecision(PublicShellStatus? status)
+    {
+        return status is { PublicIpcReady: true, ProcessId: > 0 };
+    }
+
+    private static async Task<PublicShellStatus?> TryGetExistingHostStatusAsync(
+        LanMountainDesktopIpcClient ipcClient,
+        TimeSpan timeout)
+    {
+        try
+        {
+            var connected = ipcClient.IsConnected ||
+                            await TryConnectToPublicIpcAsync(ipcClient, timeout).ConfigureAwait(false);
+            if (!connected)
+            {
+                return null;
+            }
+
+            var shellProxy = ipcClient.CreateProxy<IPublicShellControlService>();
+            return await shellProxy.GetShellStatusAsync().ConfigureAwait(false);
+        }
+        catch (Exception ex)
+        {
+            Logger.Info($"Existing host status probe did not complete: {ex.Message}");
+            return null;
+        }
+    }
+
+    private static async Task<ExistingHostBehaviorResult> ApplyExistingHostBehaviorAsync(
+        LanMountainDesktopIpcClient ipcClient,
+        MultiInstanceLaunchBehavior behavior,
+        PublicShellStatus status)
+    {
+        try
+        {
+            var shellProxy = ipcClient.CreateProxy<IPublicShellControlService>();
+            return behavior switch
+            {
+                MultiInstanceLaunchBehavior.OpenDesktopSilently => await ActivateExistingHostForBehaviorAsync(
+                    shellProxy,
+                    showLauncherNotice: false,
+                    successCode: "existing_host_activated",
+                    successMessage: "Launcher activated the existing desktop instance.",
+                    failureCode: "existing_host_activation_failed").ConfigureAwait(false),
+
+                MultiInstanceLaunchBehavior.NotifyAndOpenDesktop => await ActivateExistingHostForBehaviorAsync(
+                    shellProxy,
+                    showLauncherNotice: true,
+                    successCode: "existing_host_activated_with_notice",
+                    successMessage: "Launcher activated the existing desktop instance and showed the repeated-launch notice.",
+                    failureCode: "existing_host_activation_failed").ConfigureAwait(false),
+
+                MultiInstanceLaunchBehavior.PromptOnly => await ShowPromptOnlyExistingHostAsync(
+                    shellProxy,
+                    status).ConfigureAwait(false),
+
+                MultiInstanceLaunchBehavior.RestartApp => await RestartExistingHostAsync(shellProxy).ConfigureAwait(false),
+
+                _ => await ActivateExistingHostForBehaviorAsync(
+                    shellProxy,
+                    showLauncherNotice: true,
+                    successCode: "existing_host_activated_with_notice",
+                    successMessage: "Launcher activated the existing desktop instance and showed the repeated-launch notice.",
+                    failureCode: "existing_host_activation_failed").ConfigureAwait(false)
+            };
+        }
+        catch (Exception ex)
+        {
+            Logger.Warn($"Failed to apply multi-instance behavior '{behavior}': {ex.Message}");
+            return new ExistingHostBehaviorResult(
+                false,
+                "multi_instance_behavior_failed",
+                $"Failed to apply multi-instance behavior '{behavior}': {ex.Message}",
+                null);
+        }
+    }
+
+    private static async Task<ExistingHostBehaviorResult> ActivateExistingHostForBehaviorAsync(
+        IPublicShellControlService shellProxy,
+        bool showLauncherNotice,
+        string successCode,
+        string successMessage,
+        string failureCode)
+    {
+        var activation = await shellProxy.ActivateMainWindowWithStatusAsync().ConfigureAwait(false);
+        var success = activation.Accepted || IsRecoverableActivationFailure(activation);
+        if (showLauncherNotice && success)
+        {
+            var promptResult = await ShowMultiInstancePromptAsync(activation.Status).ConfigureAwait(false);
+            if (promptResult == MultiInstancePromptResult.OpenDesktop)
+            {
+                activation = await shellProxy.ActivateMainWindowWithStatusAsync().ConfigureAwait(false);
+            }
+        }
+
+        return new ExistingHostBehaviorResult(
+            success,
+            activation.Accepted ? successCode : success ? "existing_host_startup_pending" : failureCode,
+            activation.Accepted ? successMessage : activation.Message,
+            activation);
+    }
+
+    private static async Task<ExistingHostBehaviorResult> RestartExistingHostAsync(
+        IPublicShellControlService shellProxy)
+    {
+        var accepted = await shellProxy.RestartAsync().ConfigureAwait(false);
+        return new ExistingHostBehaviorResult(
+            accepted,
+            accepted ? "existing_host_restart_requested" : "existing_host_restart_failed",
+            accepted
+                ? "Launcher requested the existing desktop instance to restart."
+                : "Launcher could not request restart from the existing desktop instance.",
+            null);
+    }
+
+    private static async Task<ExistingHostBehaviorResult> ShowPromptOnlyExistingHostAsync(
+        IPublicShellControlService shellProxy,
+        PublicShellStatus status)
+    {
+        var promptResult = await ShowMultiInstancePromptAsync(status).ConfigureAwait(false);
+
+        if (promptResult == MultiInstancePromptResult.OpenDesktop)
+        {
+            return await ActivateExistingHostForBehaviorAsync(
+                shellProxy,
+                showLauncherNotice: false,
+                successCode: "existing_host_activated_from_prompt",
+                successMessage: "Launcher activated the existing desktop instance from the prompt.",
+                failureCode: "existing_host_activation_failed").ConfigureAwait(false);
+        }
+
+        return new ExistingHostBehaviorResult(
+            true,
+            "existing_host_prompt_only",
+            "Launcher showed the repeated-launch prompt and did not open the desktop automatically.",
+            null);
+    }
+
+    private static async Task<MultiInstancePromptResult> ShowMultiInstancePromptAsync(PublicShellStatus status)
+    {
+        return await Dispatcher.UIThread.InvokeAsync(async () =>
+        {
+            var prompt = new MultiInstancePromptWindow();
+            prompt.SetDetails(status.ProcessId, status.ShellState);
+            prompt.Show();
+            return await prompt.WaitForChoiceAsync().ConfigureAwait(true);
+        });
+    }
+
    private static async Task<PublicShellActivationResult?> TryActivateExistingHostWithStatusAsync(
        LanMountainDesktopIpcClient ipcClient,
        TimeSpan timeout)
@@ -1507,7 +1573,7 @@ internal sealed class LauncherFlowCoordinator
        }
        catch (Exception ex)
        {
-            Logger.Warn($"Existing host activation probe failed: {ex.Message}");
+            Logger.Info($"Existing host activation probe did not complete: {ex.Message}");
            return null;
        }
    }
@@ -1541,7 +1607,7 @@ internal sealed class LauncherFlowCoordinator
            : null;
    }

-    private static bool IsRecoverableActivationFailure(PublicShellActivationResult activation)
+    internal static bool IsRecoverableActivationFailure(PublicShellActivationResult activation)
    {
        if (activation.Accepted)
        {
@@ -1560,6 +1626,12 @@ internal sealed class LauncherFlowCoordinator
                string.Equals(activation.Code, "startup_pending", StringComparison.OrdinalIgnoreCase));
    }

+    internal static bool IsSuccessfulActivationExitCode(int exitCode) =>
+        exitCode == HostExitCodes.SecondaryActivationSucceeded;
+
+    internal static bool IsFailedActivationExitCode(int exitCode) =>
+        exitCode is HostExitCodes.SecondaryActivationFailed or HostExitCodes.RestartLockNotAcquired;
+
    private static async Task<PublicShellStatus?> TryGetPublicShellStatusAsync(
        LanMountainDesktopIpcClient ipcClient)
    {
@@ -1759,6 +1831,12 @@ internal sealed class LauncherFlowCoordinator
                plan is null ? null : HostLaunchPlanBuilder.FormatArgumentsForLog(plan.Arguments));
    }

+    private sealed record ExistingHostBehaviorResult(
+        bool Success,
+        string Code,
+        string Message,
+        PublicShellActivationResult? ActivationResult);
+
    private sealed record HostLaunchOutcome(
        LauncherResult Result,
        Process? Process,
--- a/LanMountainDesktop.Launcher/Views/ErrorWindow.axaml
+++ b/LanMountainDesktop.Launcher/Views/ErrorWindow.axaml
@@ -3,16 +3,20 @@
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:views="clr-namespace:LanMountainDesktop.Launcher.Views"
+        xmlns:ui="using:FluentAvalonia.UI.Controls"
+        xmlns:fi="using:FluentIcons.Avalonia"
        mc:Ignorable="d"
        x:Class="LanMountainDesktop.Launcher.Views.ErrorWindow"
        x:DataType="views:ErrorWindow"
        Title="LanMountain Desktop"
-        Width="560"
-        Height="320"
+        Width="760"
+        Height="460"
+        MinWidth="640"
+        MinHeight="420"
        CanResize="False"
        WindowStartupLocation="CenterScreen"
-        Background="#111318"
-        TransparencyLevelHint="None"
+        Background="{DynamicResource SolidBackgroundFillColorBaseBrush}"
+        TransparencyLevelHint="Mica, AcrylicBlur, None"
        Icon="/Assets/logo.ico">
  <Design.DataContext>
    <views:ErrorWindow />
@@ -20,79 +24,128 @@

  <Grid RowDefinitions="*,Auto">
    <Grid Grid.Row="0"
-          Margin="24"
+          Margin="28,24,28,20"
+          RowDefinitions="Auto,Auto,*"
          ColumnDefinitions="Auto,*">
      <Border x:Name="ErrorIconBorder"
              Grid.Column="0"
-              Width="52"
-              Height="52"
-              Margin="0,4,18,0"
-              Background="#2B161A"
-              CornerRadius="26"
+              Width="56"
+              Height="56"
+              Margin="0,0,18,0"
+              Background="{DynamicResource SystemFillColorCriticalBackgroundBrush}"
+              CornerRadius="28"
              VerticalAlignment="Top">
-        <TextBlock Text="!"
-                   FontSize="24"
-                   FontWeight="Bold"
-                   Foreground="#FFB4AB"
-                   HorizontalAlignment="Center"
-                   VerticalAlignment="Center" />
+        <fi:SymbolIcon Symbol="ErrorCircle"
+                       IconVariant="Regular"
+                       FontSize="28"
+                       Foreground="{DynamicResource SystemFillColorCriticalBrush}"
+                       HorizontalAlignment="Center"
+                       VerticalAlignment="Center" />
      </Border>

      <StackPanel Grid.Column="1"
-                  Spacing="10">
+                  Spacing="8">
        <TextBlock x:Name="TitleText"
                   Text="Launcher could not confirm startup"
-                   FontSize="20"
+                   FontSize="22"
                   FontWeight="SemiBold"
-                   Foreground="#F6F7FB"
+                   Foreground="{DynamicResource TextFillColorPrimaryBrush}"
                   TextWrapping="Wrap" />

        <TextBlock x:Name="ErrorMessageText"
                   Text="LanMountain Desktop did not reach the expected startup state."
                   FontSize="14"
-                   Foreground="#D2D7E1"
+                   Foreground="{DynamicResource TextFillColorSecondaryBrush}"
                   TextWrapping="Wrap"
                   LineHeight="22" />
-
-        <TextBlock x:Name="SuggestionText"
-                   Text="You can inspect logs, retry when the old process is gone, or reactivate the current instance."
-                   FontSize="13"
-                   Foreground="#9BA5B7"
-                   TextWrapping="Wrap"
-                   LineHeight="20" />
      </StackPanel>
+
+      <ui:FAInfoBar x:Name="SuggestionInfoBar"
+                    Grid.Row="1"
+                    Grid.Column="0"
+                    Grid.ColumnSpan="2"
+                    Margin="0,20,0,14"
+                    IsOpen="True"
+                    IsClosable="False"
+                    Severity="Warning"
+                    Title="Startup recovery"
+                    Message="You can inspect logs, wait for the current process, or activate the running desktop instance.">
+        <ui:FAInfoBar.IconSource>
+          <ui:FAFontIconSource Glyph="&#xF0288;"
+                               FontFamily="avares://fluenticons.resources.avalonia/Assets#Seagull Fluent Icons" />
+        </ui:FAInfoBar.IconSource>
+      </ui:FAInfoBar>
+
+      <Expander Grid.Row="2"
+                Grid.Column="0"
+                Grid.ColumnSpan="2"
+                Header="Diagnostic details"
+                IsExpanded="True">
+        <TextBox x:Name="ErrorDetailsTextBox"
+                 Margin="0,10,0,0"
+                 MinHeight="150"
+                 MaxHeight="190"
+                 AcceptsReturn="True"
+                 TextWrapping="Wrap"
+                 IsReadOnly="True"
+                 BorderThickness="0"
+                 FontSize="12"
+                 Text="Stage: launch&#x0a;Code: unknown"
+                 VerticalContentAlignment="Top" />
+      </Expander>
    </Grid>

    <Border Grid.Row="1"
-            Padding="24,16"
-            Background="#171A21">
-      <Grid ColumnDefinitions="*,Auto,Auto,Auto"
-            ColumnSpacing="8">
-        <Button x:Name="OpenLogButton"
-                Grid.Column="0"
-                Content="Open Logs"
-                MinWidth="108"
-                Height="34"
-                HorizontalAlignment="Left" />
+            Padding="18,14"
+            Background="{DynamicResource LayerOnMicaBaseAltFillColorDefaultBrush}">
+      <Grid ColumnDefinitions="Auto,*,Auto"
+            ColumnSpacing="12">
+        <StackPanel Grid.Column="0"
+                    Orientation="Horizontal"
+                    Spacing="8">
+          <Button x:Name="OpenLogButton"
+                  MinWidth="112"
+                  Height="34">
+            <StackPanel Orientation="Horizontal" Spacing="6">
+              <fi:SymbolIcon Symbol="FolderOpen" IconVariant="Regular" FontSize="16"/>
+              <TextBlock Text="Open Logs"/>
+            </StackPanel>
+          </Button>

-        <Button x:Name="SecondaryActionButton"
-                Grid.Column="1"
-                Content="Wait"
-                MinWidth="108"
-                Height="34"
-                IsVisible="False" />
+          <Button x:Name="CopyDetailsButton"
+                  MinWidth="100"
+                  Height="34">
+            <StackPanel Orientation="Horizontal" Spacing="6">
+              <fi:SymbolIcon Symbol="Copy" IconVariant="Regular" FontSize="16"/>
+              <TextBlock Text="Copy"/>
+            </StackPanel>
+          </Button>
+        </StackPanel>

-        <Button x:Name="ExitButton"
-                Grid.Column="2"
-                Content="Exit"
-                MinWidth="90"
-                Height="34" />
+        <StackPanel Grid.Column="2"
+                    Orientation="Horizontal"
+                    Spacing="8">
+          <Button x:Name="SecondaryActionButton"
+                  Content="Wait"
+                  MinWidth="96"
+                  Height="34"
+                  IsVisible="False" />

-        <Button x:Name="PrimaryActionButton"
-                Grid.Column="3"
-                Content="Retry"
-                MinWidth="108"
-                Height="34" />
+          <Button x:Name="ExitButton"
+                  MinWidth="92"
+                  Height="34">
+            <StackPanel Orientation="Horizontal" Spacing="6">
+              <fi:SymbolIcon Symbol="Dismiss" IconVariant="Regular" FontSize="16"/>
+              <TextBlock Text="Exit"/>
+            </StackPanel>
+          </Button>
+
+          <Button x:Name="PrimaryActionButton"
+                  Classes="accent"
+                  Content="Retry"
+                  MinWidth="112"
+                  Height="34" />
+        </StackPanel>
      </Grid>
    </Border>
  </Grid>
--- a/LanMountainDesktop.Launcher/Views/ErrorWindow.axaml.cs
+++ b/LanMountainDesktop.Launcher/Views/ErrorWindow.axaml.cs
@@ -1,8 +1,10 @@
 using System.Diagnostics;
 using Avalonia.Controls;
 using Avalonia.Input;
+using Avalonia.Input.Platform;
 using Avalonia.Interactivity;
 using Avalonia.Markup.Xaml;
+using FluentAvalonia.UI.Controls;
 using LanMountainDesktop.Launcher.Services;

 namespace LanMountainDesktop.Launcher.Views;
@@ -33,9 +35,21 @@ public partial class ErrorWindow : Window

    public void SetErrorMessage(string message)
    {
+        var normalizedMessage = string.IsNullOrWhiteSpace(message)
+            ? "LanMountain Desktop did not reach the expected startup state."
+            : message.Trim();
+        var firstLine = normalizedMessage
+            .Split(['\r', '\n'], StringSplitOptions.RemoveEmptyEntries)
+            .FirstOrDefault() ?? normalizedMessage;
+
        if (this.FindControl<TextBlock>("ErrorMessageText") is { } errorText)
        {
-            errorText.Text = message;
+            errorText.Text = firstLine;
+        }
+
+        if (this.FindControl<TextBox>("ErrorDetailsTextBox") is { } detailsTextBox)
+        {
+            detailsTextBox.Text = normalizedMessage;
        }
    }

@@ -120,6 +134,11 @@ public partial class ErrorWindow : Window
        {
            openLogButton.Click += OnOpenLogClick;
        }
+
+        if (this.FindControl<Button>("CopyDetailsButton") is { } copyDetailsButton)
+        {
+            copyDetailsButton.Click += OnCopyDetailsClick;
+        }
    }

    private void ApplyActionLayout(
@@ -138,9 +157,9 @@ public partial class ErrorWindow : Window
            titleText.Text = title;
        }

-        if (this.FindControl<TextBlock>("SuggestionText") is { } suggestionText)
+        if (this.FindControl<FAInfoBar>("SuggestionInfoBar") is { } suggestionInfoBar)
        {
-            suggestionText.Text = suggestion;
+            suggestionInfoBar.Message = suggestion;
        }

        if (this.FindControl<Button>("PrimaryActionButton") is { } primaryButton)
@@ -243,6 +262,28 @@ public partial class ErrorWindow : Window
        await Task.CompletedTask;
    }

+    private async void OnCopyDetailsClick(object? sender, RoutedEventArgs e)
+    {
+        try
+        {
+            var details = this.FindControl<TextBox>("ErrorDetailsTextBox")?.Text;
+            if (string.IsNullOrWhiteSpace(details))
+            {
+                details = this.FindControl<TextBlock>("ErrorMessageText")?.Text;
+            }
+
+            var clipboard = TopLevel.GetTopLevel(this)?.Clipboard;
+            if (clipboard is not null && !string.IsNullOrWhiteSpace(details))
+            {
+                await clipboard.SetTextAsync(details);
+            }
+        }
+        catch (Exception ex)
+        {
+            Debug.WriteLine($"[ErrorWindow] Failed to copy diagnostics: {ex}");
+        }
+    }
+
    private void ScanDevPaths()
    {
        var executable = OperatingSystem.IsWindows() ? "LanMountainDesktop.exe" : "LanMountainDesktop";
--- a/LanMountainDesktop.Launcher/Views/MultiInstancePromptWindow.axaml
+++ b/LanMountainDesktop.Launcher/Views/MultiInstancePromptWindow.axaml
@@ -0,0 +1,124 @@
+<Window xmlns="https://github.com/avaloniaui"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+        xmlns:views="clr-namespace:LanMountainDesktop.Launcher.Views"
+        xmlns:ui="using:FluentAvalonia.UI.Controls"
+        xmlns:fi="using:FluentIcons.Avalonia"
+        mc:Ignorable="d"
+        x:Class="LanMountainDesktop.Launcher.Views.MultiInstancePromptWindow"
+        x:DataType="views:MultiInstancePromptWindow"
+        Title="LanMountain Desktop"
+        Width="620"
+        Height="360"
+        MinWidth="560"
+        MinHeight="330"
+        CanResize="False"
+        WindowStartupLocation="CenterScreen"
+        Background="{DynamicResource SolidBackgroundFillColorBaseBrush}"
+        TransparencyLevelHint="Mica, AcrylicBlur, None"
+        Icon="/Assets/logo.ico">
+  <Design.DataContext>
+    <views:MultiInstancePromptWindow />
+  </Design.DataContext>
+
+  <Grid RowDefinitions="*,Auto">
+    <Grid Grid.Row="0"
+          Margin="28,26,28,20"
+          RowDefinitions="Auto,Auto,*"
+          ColumnDefinitions="Auto,*">
+      <Border Grid.Column="0"
+              Width="52"
+              Height="52"
+              Margin="0,0,18,0"
+              Background="{DynamicResource SystemFillColorAttentionBackgroundBrush}"
+              CornerRadius="26"
+              VerticalAlignment="Top">
+        <fi:SymbolIcon Symbol="Desktop"
+                       IconVariant="Regular"
+                       FontSize="26"
+                       Foreground="{DynamicResource SystemFillColorAttentionBrush}"
+                       HorizontalAlignment="Center"
+                       VerticalAlignment="Center" />
+      </Border>
+
+      <StackPanel Grid.Column="1"
+                  Spacing="8">
+        <TextBlock Text="LanMountain Desktop is already running"
+                   FontSize="22"
+                   FontWeight="SemiBold"
+                   Foreground="{DynamicResource TextFillColorPrimaryBrush}"
+                   TextWrapping="Wrap" />
+        <TextBlock x:Name="MessageText"
+                   Text="Launcher found an existing desktop instance and did not start another process."
+                   FontSize="14"
+                   Foreground="{DynamicResource TextFillColorSecondaryBrush}"
+                   TextWrapping="Wrap"
+                   LineHeight="22" />
+      </StackPanel>
+
+      <ui:FAInfoBar Grid.Row="1"
+                    Grid.Column="0"
+                    Grid.ColumnSpan="2"
+                    Margin="0,22,0,14"
+                    IsOpen="True"
+                    IsClosable="False"
+                    Severity="Informational"
+                    Title="Repeated launch"
+                    Message="Your current setting is to show this prompt without opening the desktop automatically.">
+        <ui:FAInfoBar.IconSource>
+          <ui:FAFontIconSource Glyph="&#xF0288;"
+                               FontFamily="avares://fluenticons.resources.avalonia/Assets#Seagull Fluent Icons" />
+        </ui:FAInfoBar.IconSource>
+      </ui:FAInfoBar>
+
+      <TextBlock x:Name="DetailsText"
+                 Grid.Row="2"
+                 Grid.Column="0"
+                 Grid.ColumnSpan="2"
+                 FontSize="12"
+                 Foreground="{DynamicResource TextFillColorSecondaryBrush}"
+                 TextWrapping="Wrap"
+                 Text="No second Host process was created." />
+    </Grid>
+
+    <Border Grid.Row="1"
+            Padding="18,14"
+            Background="{DynamicResource LayerOnMicaBaseAltFillColorDefaultBrush}">
+      <Grid ColumnDefinitions="*,Auto">
+        <Button x:Name="CopyDetailsButton"
+                Grid.Column="0"
+                MinWidth="104"
+                Height="34"
+                HorizontalAlignment="Left">
+          <StackPanel Orientation="Horizontal" Spacing="6">
+            <fi:SymbolIcon Symbol="Copy" IconVariant="Regular" FontSize="16"/>
+            <TextBlock Text="Copy"/>
+          </StackPanel>
+        </Button>
+
+        <StackPanel Grid.Column="1"
+                    Orientation="Horizontal"
+                    Spacing="8">
+          <Button x:Name="CloseButton"
+                  MinWidth="92"
+                  Height="34">
+            <StackPanel Orientation="Horizontal" Spacing="6">
+              <fi:SymbolIcon Symbol="Dismiss" IconVariant="Regular" FontSize="16"/>
+              <TextBlock Text="Close"/>
+            </StackPanel>
+          </Button>
+          <Button x:Name="OpenDesktopButton"
+                  Classes="accent"
+                  MinWidth="136"
+                  Height="34">
+            <StackPanel Orientation="Horizontal" Spacing="6">
+              <fi:SymbolIcon Symbol="ArrowRight" IconVariant="Regular" FontSize="16"/>
+              <TextBlock Text="Open desktop"/>
+            </StackPanel>
+          </Button>
+        </StackPanel>
+      </Grid>
+    </Border>
+  </Grid>
+</Window>
--- a/LanMountainDesktop.Launcher/Views/MultiInstancePromptWindow.axaml.cs
+++ b/LanMountainDesktop.Launcher/Views/MultiInstancePromptWindow.axaml.cs
@@ -0,0 +1,76 @@
+using Avalonia.Controls;
+using Avalonia.Input.Platform;
+using Avalonia.Interactivity;
+using Avalonia.Markup.Xaml;
+
+namespace LanMountainDesktop.Launcher.Views;
+
+public partial class MultiInstancePromptWindow : Window
+{
+    private readonly TaskCompletionSource<MultiInstancePromptResult> _completionSource =
+        new(TaskCreationOptions.RunContinuationsAsynchronously);
+    private string _details = "LanMountain Desktop is already running.";
+
+    public MultiInstancePromptWindow()
+    {
+        AvaloniaXamlLoader.Load(this);
+        Loaded += OnLoaded;
+        Closed += (_, _) => _completionSource.TrySetResult(MultiInstancePromptResult.Close);
+    }
+
+    public Task<MultiInstancePromptResult> WaitForChoiceAsync() => _completionSource.Task;
+
+    public void SetDetails(int processId, string shellState)
+    {
+        _details = $"Existing host PID: {processId}\nShell state: {shellState}\nNo second Host process was created.";
+
+        if (this.FindControl<TextBlock>("DetailsText") is { } detailsText)
+        {
+            detailsText.Text = _details;
+        }
+    }
+
+    private void OnLoaded(object? sender, RoutedEventArgs e)
+    {
+        if (this.FindControl<Button>("CloseButton") is { } closeButton)
+        {
+            closeButton.Click += (_, _) => Complete(MultiInstancePromptResult.Close);
+        }
+
+        if (this.FindControl<Button>("OpenDesktopButton") is { } openDesktopButton)
+        {
+            openDesktopButton.Click += (_, _) => Complete(MultiInstancePromptResult.OpenDesktop);
+        }
+
+        if (this.FindControl<Button>("CopyDetailsButton") is { } copyDetailsButton)
+        {
+            copyDetailsButton.Click += OnCopyDetailsClick;
+        }
+    }
+
+    private void Complete(MultiInstancePromptResult result)
+    {
+        _completionSource.TrySetResult(result);
+        Close();
+    }
+
+    private async void OnCopyDetailsClick(object? sender, RoutedEventArgs e)
+    {
+        try
+        {
+            if (TopLevel.GetTopLevel(this)?.Clipboard is IClipboard clipboard)
+            {
+                await clipboard.SetTextAsync(_details);
+            }
+        }
+        catch
+        {
+        }
+    }
+}
+
+public enum MultiInstancePromptResult
+{
+    Close,
+    OpenDesktop
+}