TRAX: toggle refreshes the status light on play/pause, terse scrolling error on failure, fixed quiet volume (0.5), ITraxPlayer seam. Deadlocked test fixed via RunAsync (was GetAwaiter().GetResult on the UI thread). 416 pass / 1 known env flake. Refs: https://learn.microsoft.com/en-us/dotnet/standard/asynchronous-programming-patterns/async-programming-model-tap#dont-block-on-async-code (Unit: trax toggle)

ViewModels/MainViewModel.Trax.cs

@@ -10,9 +10,13 @@ namespace ytLive.ViewModels;
 public partial class MainViewModel
 {
     // TRAX (TASK 9): free background music at a fixed quiet volume, playing to
-    // the default device so it rides the loopback into the stream. The dot is
-    // red (no track) / yellow (loaded, stopped) / green (playing).
-    private readonly MusicPlayer _musicPlayer = new();
+    // the default device so it rides the loopback into the stream. The button is a
+    // toggle; its dot is green (playing) / red (error — a terse message scrolls in the
+    // desktop-audio bar) / gray (idle or no track).
+    private readonly ITraxPlayer _musicPlayer = TraxPlayerOverride?.Invoke() ?? new MusicPlayer();
+
+    /// <summary>Test seam: a fake TRAX player (no audio device). Set before constructing.</summary>
+    internal static Func<ITraxPlayer>? TraxPlayerOverride { get; set; }
     private Music? _music;
     private bool _musicPlaying;
     private bool _traxLoadInFlight;
@@ -46,13 +50,18 @@ public partial class MainViewModel
     public string? TraxError
     {
         get => _traxError;
-        private set => SetProperty(ref _traxError, value);
+        private set
+        {
+            if (SetProperty(ref _traxError, value))
+                OnPropertyChanged(nameof(TraxDotBrush));
+        }
     }
 
-    /// <summary>TRAX status dot: green = playing, gray = idle or no track.</summary>
+    /// <summary>TRAX status dot: green = playing, red = the last load/play failed (the
+    /// message scrolls in the desktop-audio bar), gray = idle or no track.</summary>
     public SolidColorBrush TraxDotBrush => _musicPlayer.IsPlaying
         ? BarOnBrush
-        : TraxIdleBrush;
+        : TraxError != null ? BarOffBrush : TraxIdleBrush;
 
     public string TraxToolTip => _musicPlayer.TrackName != null
         ? $"TRAX: {_musicPlayer.TrackName} — left-click {(IsMusicPlaying ? "pauses" : "plays")}, right-click chooses a track"
@@ -82,8 +91,15 @@ public partial class MainViewModel
         if (dialog.ShowDialog() != true)
             return;
 
-        _music = new Music { TrackPath = dialog.FileName, IsEnabled = true };
-        LoadTrax(play: true);
+        _ = SetTraxTrackAsync(dialog.FileName, play: true);
+    }
+
+    /// <summary>Points TRAX at a track and loads it (optionally playing). The picker's path;
+    /// internal so tests can load a track without the file dialog.</summary>
+    internal Task SetTraxTrackAsync(string path, bool play)
+    {
+        _music = new Music { TrackPath = path, IsEnabled = true };
+        return LoadTraxAsync(play);
     }
 
     /// <summary>Probes a path off the UI thread and gives up at <paramref name="budget"/>.
@@ -128,20 +144,20 @@ public partial class MainViewModel
                 UpdateTraxLocalGain();
                 music.IsEnabled = true;
                 TraxError = null;
-                if (play)
-                    _musicPlayer.Play();
                 loaded = true;
+                if (play)
+                    PlayTrax();
             }
             else
             {
                 AppLog.Write($"TRAX: '{path}' unreachable within {TraxProbeBudget.TotalSeconds:0}s — not loading");
-                TraxError = $"Can't reach {Path.GetFileName(path)} — is the drive on?";
+                TraxError = "TRAX: can't reach the track";
             }
         }
         catch (Exception ex)
         {
             AppLog.Write($"TRAX: failed to load '{music.TrackPath}': {ex.Message}");
-            TraxError = $"Failed to load: {Path.GetFileName(music.TrackPath)} — {ex.Message}";
+            TraxError = "TRAX: can't load the track";
         }
         finally
         {
@@ -152,16 +168,45 @@ public partial class MainViewModel
         }
     }
 
-    private void PlayTrax() => _musicPlayer.Play();
+    // Both toggle halves MUST refresh the UI: the dot and IsMusicPlaying used to update only
+    // on load/playback-ended, so pressing TRAX started the music with the light still off —
+    // and because IsMusicPlaying stayed false, the next press tried to play again instead of
+    // pausing (2026-10-09 bug).
+    private void PlayTrax()
+    {
+        try
+        {
+            _musicPlayer.Play();
+            TraxError = _musicPlayer.IsPlaying ? null : "TRAX: no audio output";
+            if (TraxError != null) AppLog.Write("TRAX: Play() returned but playback did not start (no output device?)");
+        }
+        catch (Exception ex)
+        {
+            AppLog.Write($"TRAX: play failed: {ex.Message}");
+            TraxError = "TRAX: can't play the track";
+        }
+        RefreshTraxUi();
+    }
 
-    private void PauseTrax() => _musicPlayer.Pause();
+    private void PauseTrax()
+    {
+        try
+        {
+            _musicPlayer.Pause();
+        }
+        catch (Exception ex)
+        {
+            AppLog.Write($"TRAX: pause failed: {ex.Message}");
+        }
+        RefreshTraxUi();
+    }
 
     private void OnTraxPlaybackEnded(Exception? ex)
     {
         if (ex != null)
         {
             AppLog.Write($"TRAX: playback error: {ex.Message}");
-            TraxError = $"Playback error: {ex.Message}";
+            TraxError = "TRAX: playback stopped";
         }
         RefreshTraxUi();
     }