Webcam resource validation + first-frame proof + CameraConflictProbe: MediaCaptureFrameSource validates post-init (VideoDeviceId match, stream properties, reader StartAsync status), subscribes Failed/CameraStreamStateChanged → SourceFailed; CameraManager.AcquireAsync requires first-frame proof (4s timeout) — silent empty box impossible; MainViewModel surfaces WebcamError chip + MessageBox with suspect-app names; fallback ladder VideoPreview → VideoRecord; 19041 SDK projection gaps documented; AGENTS.md + schema.md + HANDOFF.md memory-map rules added; 81 tests passing, 0 warnings

ViewModels/MainViewModel.cs

@@ -74,7 +74,7 @@ public class MainViewModel : ViewModelBase
     private readonly ICameraEnumerator _cameraEnumerator;
     private readonly CameraManager _cameraManager;
     private Webcam? _webcam;
-
+    private string? _webcamError;
     private readonly IMicrophoneEnumerator _microphoneEnumerator;
 
     // Screen backdrop: a permanent live capture (desktop/game) that every scene
@@ -171,6 +171,17 @@ public class MainViewModel : ViewModelBase
     /// <summary>Shows the mirror / clip-shape row in the source chip when a webcam is selected.</summary>
     public bool IsWebcamSelected => SelectedElement is WebcamSceneConfig;
 
+    /// <summary>
+    /// The reason the webcam feed is down (device in use, offline, locked, no frames),
+    /// or null when it's alive. Surfaced as a red chip so a dead camera is never a
+    /// silent empty box. Cleared the moment a real frame arrives.
+    /// </summary>
+    public string? WebcamError
+    {
+        get => _webcamError;
+        private set => SetProperty(ref _webcamError, value);
+    }
+
     public StreamStatus StreamStatus
     {
         get => _streamStatus;
@@ -783,6 +794,7 @@ public class MainViewModel : ViewModelBase
             id => new MediaCaptureFrameSource(id),
             System.Windows.Application.Current?.Dispatcher);
         _cameraManager.PreviewBitmapChanged += OnCameraPreviewBitmapChanged;
+        _cameraManager.CameraFailed += OnCameraFailed;
 
         _microphoneEnumerator = new WinRtMicrophoneEnumerator();
 
@@ -921,15 +933,25 @@ public class MainViewModel : ViewModelBase
     }
 
     // CameraManager creates the shared WriteableBitmap on the UI thread at the
-    // device's frame size; every scene's webcam config picks it up from here.
+    // device's frame size; every scene's webcam config picks it up from here. A
+    // bitmap means a real frame arrived — the camera is provably alive.
     private void OnCameraPreviewBitmapChanged(string deviceId, WriteableBitmap bitmap)
     {
         if (_webcam?.DeviceId != deviceId) return;
+        WebcamError = null;
         foreach (var scene in Scenes)
             foreach (var config in scene.Elements.OfType<WebcamSceneConfig>())
                 config.VideoImageSource = bitmap;
     }
 
+    // The camera failed to start or died asynchronously (in use, offline, locked,
+    // no frames within the proof timeout) — surface it instead of a silent box.
+    private void OnCameraFailed(string deviceId, string message)
+    {
+        if (_webcam?.DeviceId != deviceId) return;
+        WebcamError = $"Webcam offline: {message}";
+    }
+
     // ─── Screen backdrop capture (live desktop/game) ───
 
     private const string MonitorKeyPrefix = "monitor:";
@@ -1348,7 +1370,7 @@ public class MainViewModel : ViewModelBase
         if (!started)
         {
             MessageBox.Show(
-                "Couldn't start that camera. It may be in use by another app, or Windows camera access may be turned off.",
+                WebcamError ?? "Couldn't start that camera. It may be in use by another app, or Windows camera access may be turned off.",
                 "ytLlive", MessageBoxButton.OK, MessageBoxImage.Warning);
         }
     }
@@ -1372,7 +1394,7 @@ public class MainViewModel : ViewModelBase
             if (!started)
             {
                 MessageBox.Show(
-                    "Couldn't start that camera. It may be in use by another app, or Windows camera access may be turned off.",
+                    WebcamError ?? "Couldn't start that camera. It may be in use by another app, or Windows camera access may be turned off.",
                     "ytLlive", MessageBoxButton.OK, MessageBoxImage.Warning);
                 break;
             }