fix(wpf): de-elevate via runas env-var marker (CLI arg breaks runas /trustlevel)

The earlier de-elevation attempts failed because runas /trustlevel:0x20000 rejects any args after the program path (returns exit code 1 silently). Switch the relaunch loop-guard from --relaunched CLI arg to TEAMSISO_RELAUNCHED env var, which runas inherits and propagates cleanly. Also: always demote when elevated regardless of parent (the parent==explorer heuristic was too narrow; the runas demotion is cheap enough to do unconditionally), and add a StartupTrace fallback log at %LOCALAPPDATA%\\TeamsISO\\startup-trace.log that captures every checkpoint in OnStartup so future launch failures can be diagnosed without Serilog being up.

Verified end-to-end: elevated parent (PID 47536, isAdmin=True) -> spawns runas -> medium-integrity child (PID 51228, isAdmin=False) -> NDI discovery succeeds (vm.Participants.Count=2 at +5s). The TryDeElevateAndExit now returns bool so spawn failures fall through to normal startup instead of leaving the process in a zombie state.

Opt-out: --keep-elevation CLI arg bypasses the demotion.

src/TeamsISO.App/App.xaml.cs

@@ -1,3 +1,4 @@
+using System.IO;
 using System.Runtime.InteropServices;
 using System.Windows;
 using System.Windows.Interop;
@@ -81,91 +82,122 @@ public partial class App : Application
 
     protected override async void OnStartup(StartupEventArgs e)
     {
-        base.OnStartup(e);
-
-        // Re-launch detection: when explorer.exe is the parent AND we're elevated,
-        // NDI Find returns zero sources — reproducible on this user's box and
-        // suspected to be a window-station / desktop-handle inheritance quirk
-        // that NDI's mDNS layer is sensitive to. The exact same exe spawned
-        // from any other parent (PowerShell, cmd, another non-explorer process)
-        // discovers sources fine. Re-spawn through runas /trustlevel:0x20000
-        // to drop to medium integrity and detach from explorer's process tree.
-        //
-        // We pass --relaunched on the re-spawn so we don't loop if the trustlevel
-        // demotion didn't take. CLI args that the operator passed (e.g.
-        // --apply-preset NAME) are forwarded verbatim to the relaunched child.
-        if (ShouldDeElevate(e.Args, out var relaunchArgs))
+        // RAW TRACE — captures startup BEFORE Serilog comes up. Helps diagnose
+        // launches where the Serilog log stays empty (silent file-sink failure,
+        // pre-logger crash, weird parent-spawn environment, etc.). Writes to
+        // %LOCALAPPDATA%\TeamsISO\startup-trace.log.
+        var parentName = "(unknown)";
+        try { parentName = TryGetParentProcessName() ?? "(null)"; } catch { }
+        StartupTrace.Write($"OnStartup ENTER. exe={Environment.ProcessPath} parent={parentName} args=[{string.Join(' ', e.Args)}]");
+        try
         {
-            TryDeElevateAndExit(relaunchArgs);
-            return; // Shutdown happens inside TryDeElevateAndExit if the spawn succeeds.
+            using var id = System.Security.Principal.WindowsIdentity.GetCurrent();
+            var pr = new System.Security.Principal.WindowsPrincipal(id);
+            StartupTrace.Write($"identity user={id.Name} isAdmin={pr.IsInRole(System.Security.Principal.WindowsBuiltInRole.Administrator)} integrity-token={id.User}");
+        }
+        catch (Exception ex) { StartupTrace.Write($"identity probe FAILED: {ex}"); }
+
+        base.OnStartup(e);
+        StartupTrace.Write("base.OnStartup returned");
+
+        // De-elevation check — see ShouldDeElevate doc. Trace records the decision.
+        bool deElev = false;
+        string[] relaunchArgs = e.Args;
+        try { deElev = ShouldDeElevate(e.Args, out relaunchArgs); } catch (Exception ex) { StartupTrace.Write($"ShouldDeElevate THREW: {ex}"); }
+        StartupTrace.Write($"ShouldDeElevate decision: {deElev}");
+        if (deElev)
+        {
+            var didExit = TryDeElevateAndExit(relaunchArgs);
+            if (didExit)
+            {
+                // Shutdown(0) was issued; let WPF tear us down. No more code runs.
+                return;
+            }
+            // Spawn failed — fall through to normal startup as a fallback so the
+            // operator at least sees a window. They may hit the elevated-launch
+            // bug (no participants) but that's better than nothing.
+            StartupTrace.Write("de-elevate spawn failed — falling through to normal startup as fallback");
         }
 
         // Crash diagnostics — wire the three exception channels WPF leaves open by
-        // default to a single handler that logs Fatal to Serilog (which has the
-        // rolling-daily file sink at %LOCALAPPDATA%\TeamsISO\Logs) and then shows
-        // the user a dialog with the log path so they can attach it to a bug
-        // report. We deliberately don't catch StackOverflowException or
-        // ExecutionEngineException — both are uncatchable in modern .NET; if one
-        // fires the OS Watson dialog will take it from here.
+        // default to a single handler that logs Fatal to Serilog.
         AppDomain.CurrentDomain.UnhandledException += OnAppDomainUnhandled;
         DispatcherUnhandledException += OnDispatcherUnhandled;
         System.Threading.Tasks.TaskScheduler.UnobservedTaskException += OnUnobservedTaskException;
+        StartupTrace.Write("crash handlers registered");
 
-        // Resolve and apply the theme BEFORE any window is shown so we don't
-        // paint a dark frame for one tick then flip to light (or vice versa).
-        // ThemeManager.Apply swaps Application.Resources.MergedDictionaries
-        // in place; DynamicResource refs in WildDragonTheme.xaml re-bind.
-        TeamsISO.App.Services.ThemeManager.Current.Apply();
+        try { TeamsISO.App.Services.ThemeManager.Current.Apply(); StartupTrace.Write("ThemeManager.Apply OK"); }
+        catch (Exception ex) { StartupTrace.Write($"ThemeManager.Apply THREW: {ex}"); }
 
-        // Single-instance gate. Implementation in App.Bootstrap.cs; we
-        // bail silently if another instance already owns the mutex (the
-        // existing instance gets surfaced via the bring-to-front broadcast).
-        if (!TryAcquireSingleInstance())
+        // Single-instance gate. Trace the mutex acquisition.
+        bool acquired = false;
+        try { acquired = TryAcquireSingleInstance(); } catch (Exception ex) { StartupTrace.Write($"TryAcquireSingleInstance THREW: {ex}"); }
+        StartupTrace.Write($"TryAcquireSingleInstance returned: {acquired}");
+        if (!acquired)
         {
+            StartupTrace.Write("not first instance — Shutdown(0)");
             Shutdown(0);
             return;
         }
 
         try
         {
-            // WPF host: write to both console (visible if attached) and a
-            // rolling daily file under %LOCALAPPDATA%\TeamsISO\Logs so users
-            // have something to grab when they file an issue.
+            StartupTrace.Write("Bootstrap try-block ENTER");
             _loggerFactory = EngineLogging.CreateDefault(LogLevel.Information);
+            StartupTrace.Write("EngineLogging.CreateDefault OK");
             var logger = _loggerFactory.CreateLogger<App>();
             logger.LogInformation(
                 "TeamsISO.App starting up. Build: {Version}. Process: {Pid}.",
                 typeof(App).Assembly.GetName().Version,
                 Environment.ProcessId);
+            StartupTrace.Write("Serilog first write attempted");
 
             if (!TryBootstrapNdiInterop())
             {
+                StartupTrace.Write("TryBootstrapNdiInterop returned false — Shutdown(2)");
                 Shutdown(2);
                 return;
             }
+            StartupTrace.Write("TryBootstrapNdiInterop OK");
 
             BootstrapEngine();
+            StartupTrace.Write("BootstrapEngine OK");
             var window = ConstructAndShowMainWindow();
+            StartupTrace.Write("ConstructAndShowMainWindow OK (window shown)");
             BootstrapControlSurfaceServices();
+            StartupTrace.Write("BootstrapControlSurfaceServices OK");
             BootstrapTrayIcon(window);
+            StartupTrace.Write("BootstrapTrayIcon OK");
             TryShowOnboarding(window);
+            StartupTrace.Write("TryShowOnboarding returned");
 
-            // Parse CLI args BEFORE InitializeAsync so any --apply-preset
-            // request overrides the persisted auto-apply preference cleanly.
             ApplyCommandLineArgs(e.Args);
+            StartupTrace.Write("ApplyCommandLineArgs OK");
 
+            StartupTrace.Write("about to await _viewModel.InitializeAsync");
             await _viewModel!.InitializeAsync(CancellationToken.None);
+            StartupTrace.Write("_viewModel.InitializeAsync COMPLETED");
 
             TryAutoLaunchTeams(logger);
             StartBackgroundUpdateCheck(logger);
+            StartupTrace.Write("OnStartup COMPLETE");
+
+            // 5-second post-init participant probe — tells us whether discovery
+            // is actually producing rows once the engine is up.
+            _ = Task.Run(async () =>
+            {
+                await Task.Delay(5000);
+                try
+                {
+                    var n = await Dispatcher.InvokeAsync(() => _viewModel?.Participants.Count ?? -1);
+                    StartupTrace.Write($"+5s after init: vm.Participants.Count={n}");
+                }
+                catch (Exception ex) { StartupTrace.Write($"+5s probe THREW: {ex.Message}"); }
+            });
         }
         catch (Exception ex)
         {
-            // Log the full exception (incl. stack + inner) to Serilog BEFORE the
-            // modal MessageBox fires — diagnostic logs are far more useful than a
-            // user-pasted "TeamsISO failed to start..." line when triaging a
-            // startup crash. The logger may itself have been the failure target
-            // so guard the call.
+            StartupTrace.Write($"OnStartup CATCH: {ex}");
             try { _loggerFactory?.CreateLogger<App>().LogCritical(ex, "OnStartup failed before main loop"); }
             catch { /* defensive */ }
             MessageBox.Show(
@@ -179,46 +211,60 @@ public partial class App : Application
 
     /// <summary>
     /// Returns true when we need to re-spawn ourselves with a non-elevated
-    /// medium-integrity token. This is the case when:
+    /// medium-integrity token. Rule:
     /// <list type="number">
-    ///   <item>We haven't already been relaunched (<c>--relaunched</c> guard
-    ///         prevents infinite loops if the demotion didn't take).</item>
-    ///   <item>The current process token has the Administrators group
-    ///         elevated (UAC "split-token" — admin SID is present and active).</item>
-    ///   <item>Our parent process is <c>explorer.exe</c> — that's the spawn
-    ///         path that triggers the NDI mDNS-isolation bug. Launches from
-    ///         PowerShell, cmd, or any other parent work fine even when
-    ///         elevated, so we don't need to fight them.</item>
+    ///   <item>If we've already been relaunched once (<c>--relaunched</c>
+    ///         marker present in args), DO NOT demote again. Strip the
+    ///         marker from forwardArgs so it doesn't leak further.</item>
+    ///   <item>If our token is elevated (Administrators group active),
+    ///         demote — full stop, regardless of parent.</item>
     /// </list>
+    /// <para>
+    /// The earlier "only if parent == explorer.exe" heuristic was too narrow:
+    /// the operator's broken spawn path on this dev box is double-clicking
+    /// TeamsISO.exe from an elevated File Explorer, which Windows turns into
+    /// a CreateProcess where the parent record is not always explorer (it
+    /// depends on Windows version, shell extension state, and whether the
+    /// click went through the shell namespace cache). Demoting whenever we
+    /// see an elevated token is safer and cheaper than trying to disambiguate
+    /// the spawn chain. The cost is one extra millisecond on launch + a brief
+    /// console flash from runas; the win is that NDI discovery actually works.
+    /// </para>
+    /// <para>
+    /// If you ever need to run TeamsISO elevated on purpose (debugging some
+    /// admin-only API path), pass <c>--keep-elevation</c> on the command line
+    /// to bypass this check.
+    /// </para>
     /// </summary>
+    private const string RelaunchEnvVar = "TEAMSISO_RELAUNCHED";
+
     private static bool ShouldDeElevate(string[] args, out string[] forwardArgs)
     {
         forwardArgs = args;
-        // Already relaunched once — don't loop.
-        if (Array.IndexOf(args, "--relaunched") >= 0)
+        // Already relaunched once — don't loop. The marker is an env var
+        // (NOT a CLI arg) because runas.exe /trustlevel:0x20000 fails with
+        // exit code 1 when extra args follow the program path; the env var
+        // is inherited cleanly across the runas boundary.
+        if (string.Equals(Environment.GetEnvironmentVariable(RelaunchEnvVar), "1", StringComparison.Ordinal))
         {
-            // Strip the marker so it doesn't propagate further.
-            forwardArgs = args.Where(a => a != "--relaunched").ToArray();
+            // Clear it so a future legitimately-elevated launch isn't suppressed.
+            Environment.SetEnvironmentVariable(RelaunchEnvVar, null);
             return false;
         }
-        // Not elevated — nothing to demote from.
+        // Explicit opt-out for power users.
+        if (Array.IndexOf(args, "--keep-elevation") >= 0)
+        {
+            forwardArgs = args.Where(a => a != "--keep-elevation").ToArray();
+            return false;
+        }
+        // The whole reason for the check — are we elevated?
         try
         {
             using var identity = System.Security.Principal.WindowsIdentity.GetCurrent();
             var principal = new System.Security.Principal.WindowsPrincipal(identity);
-            if (!principal.IsInRole(System.Security.Principal.WindowsBuiltInRole.Administrator))
-                return false;
+            return principal.IsInRole(System.Security.Principal.WindowsBuiltInRole.Administrator);
         }
         catch { return false; }
-        // Check parent process; if anything but explorer.exe we leave well alone.
-        try
-        {
-            var parentName = TryGetParentProcessName();
-            if (!string.Equals(parentName, "explorer", StringComparison.OrdinalIgnoreCase))
-                return false;
-        }
-        catch { return false; }
-        return true;
     }
 
     /// <summary>
@@ -247,46 +293,69 @@ public partial class App : Application
     /// Re-launch TeamsISO via <c>runas.exe /trustlevel:0x20000</c>. The
     /// trustlevel argument requests a medium-integrity restricted token —
     /// even when the caller (us) is elevated, the spawned child runs at
-    /// medium. This detaches us from explorer's spawn quirks AND from the
-    /// elevation that was tripping NDI Find. We then <see cref="Application.Shutdown(int)"/>
-    /// the current process so only the medium-integrity child remains.
-    ///
-    /// If the spawn fails for any reason (runas missing, permission denied,
-    /// etc.) we silently continue startup — the operator may still see the
-    /// "no ndi sources visible" state, but at least the app launches.
+    /// medium integrity. This sidesteps the elevation that was tripping
+    /// NDI Find. After the spawn, <see cref="Application.Shutdown(int)"/>
+    /// so only the medium-integrity child remains.
     /// </summary>
-    private void TryDeElevateAndExit(string[] forwardArgs)
+    /// <returns>true if a child was spawned and the caller should Shutdown;
+    /// false if the spawn failed and the caller should fall through to
+    /// normal (elevated) startup.</returns>
+    private bool TryDeElevateAndExit(string[] forwardArgs)
     {
         try
         {
             var exePath = System.Diagnostics.Process.GetCurrentProcess().MainModule?.FileName;
-            if (string.IsNullOrEmpty(exePath)) return; // can't relaunch what we can't find
+            if (string.IsNullOrEmpty(exePath))
+            {
+                StartupTrace.Write("de-elevate: exePath empty, giving up");
+                return false;
+            }
+            StartupTrace.Write($"de-elevate: spawning runas with target {exePath}");
 
             var quotedExe = "\"" + exePath + "\"";
-            var forwarded = string.Join(" ", forwardArgs.Select(a => "\"" + a + "\""));
-            var trustArg = string.IsNullOrEmpty(forwarded)
-                ? quotedExe + " --relaunched"
-                : quotedExe + " --relaunched " + forwarded;
+            // runas /trustlevel:0x20000 rejects any args after the program
+            // path (returns exit 1). Pass ONLY the path; relay re-launch
+            // state via the TEAMSISO_RELAUNCHED env var, which runas
+            // inherits and propagates to the spawned child.
+            // Operator CLI args (e.g. --apply-preset NAME) are not
+            // forwarded across de-elevation for the same reason; this is
+            // an acceptable tradeoff because the elevated launch was
+            // probably an Explorer double-click with no args anyway.
+
+            // Find runas.exe explicitly under System32 (the native 64-bit path).
+            var systemRunas = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.System), "runas.exe");
+            var runasPath = File.Exists(systemRunas) ? systemRunas : "runas.exe";
 
             var psi = new System.Diagnostics.ProcessStartInfo
             {
-                FileName = "runas.exe",
-                Arguments = "/trustlevel:0x20000 " + trustArg,
+                FileName = runasPath,
+                Arguments = "/trustlevel:0x20000 " + quotedExe,
                 UseShellExecute = false,
                 CreateNoWindow = true,
                 WindowStyle = System.Diagnostics.ProcessWindowStyle.Hidden,
             };
-            System.Diagnostics.Process.Start(psi);
-        }
-        catch
+            // Mark the env so the demoted child knows it's the relaunch and
+            // won't loop. runas + CreateProcess passes the parent env block
+            // to the new child by default.
+            psi.EnvironmentVariables[RelaunchEnvVar] = "1";
+            using var spawned = System.Diagnostics.Process.Start(psi);
+            if (spawned is null)
             {
-            // Relaunch failed — let normal startup proceed. Worst case the operator
-            // sees the empty-state and has to launch differently.
-            return;
+                StartupTrace.Write("de-elevate: Process.Start returned null");
+                return false;
             }
-        // Shutdown WITHOUT a value so OnExit handlers don't run a teardown for an
-        // engine that was never wired up.
+            StartupTrace.Write($"de-elevate: runas spawned as PID {spawned.Id}");
+        }
+        catch (Exception ex)
+        {
+            StartupTrace.Write($"de-elevate: spawn THREW: {ex.GetType().Name}: {ex.Message}");
+            return false;
+        }
+        // Spawn succeeded — shut ourselves down so only the medium child remains.
+        // Use Shutdown(0) to signal a clean exit (NOT a startup error).
+        StartupTrace.Write("de-elevate: calling Shutdown(0) to let runas child take over");
         Shutdown(0);
+        return true;
     }
 
     /// <summary>

src/TeamsISO.App/StartupTrace.cs

@@ -0,0 +1,40 @@
+using System.IO;
+
+namespace TeamsISO.App;
+
+/// <summary>
+/// Bare-metal startup tracer that opens, appends, and closes a file on
+/// every call. Used to capture what's happening BEFORE Serilog comes up
+/// (and to capture failures that would prevent Serilog from coming up at
+/// all). Failures here are swallowed — we never want diagnostics to crash
+/// the very thing we're trying to diagnose.
+///
+/// File lives at <c>%LOCALAPPDATA%\TeamsISO\startup-trace.log</c>. Grows
+/// without rotation; expected to be tiny since each launch writes ~20
+/// lines. Acceptable cost for catching launch-time regressions.
+/// </summary>
+internal static class StartupTrace
+{
+    private static readonly object _gate = new();
+
+    public static void Write(string message)
+    {
+        try
+        {
+            var dir = Path.Combine(
+                Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData),
+                "TeamsISO");
+            Directory.CreateDirectory(dir);
+            var path = Path.Combine(dir, "startup-trace.log");
+            var line = $"[{DateTimeOffset.Now:yyyy-MM-dd HH:mm:ss.fff}] [PID {Environment.ProcessId}] {message}{Environment.NewLine}";
+            lock (_gate)
+            {
+                File.AppendAllText(path, line);
+            }
+        }
+        catch
+        {
+            // Diagnostics must NEVER crash startup.
+        }
+    }
+}