TASK 21 slice 1 step 4: wire media into resolver + preview routing

Wire the MediaVideoSourceManager into the live pipeline (new partial
ViewModels/MainViewModel.Media.cs mirrors Webcam/Background partials):
readonly _mediaManager assigned in the core ctor (decode at 1920x1080 master,
dispatcher-coalesced preview), OnMediaPreviewBitmapChanged adopts the shared
bitmap onto every IsMediaSource with a matching MediaPath, a
Source { IsMediaSource, MediaPath } case in ResolveOutputFrame feeds
GetLatestFrame, and the manager is disposed on shutdown.

Source.DisplaySource now routes VideoImageSource for MediaSource (Source.cs),
so media previews/canvas show decoded video.

Compositor needs no change: BlitContent UniformToFill-scales any frame to the
element rect. Decoder/manager/seam derived-work mirrors ScreenCaptureManager
(citation in commit for slice-step-3).

Test (one unit for this change, mirroring the live-capture case):
Source_DisplaySource_IsVideoImageSource_WhenMediaSource. 14/14 media +
DisplaySource tests pass, build 0 warnings. Docs (TASKS/HANDOFF/ai.md) updated.

HANDOFF.md

@@ -34,13 +34,21 @@ This session:
   single shared bitmap, coalesce-to-latest). **Verified: all 10 media tests pass
   (5 manager + 3 reader + 1 decoder integration + 1), build 0 warnings.**
 
-Next (slice 1 step 4 — VM wire-in): create `_mediaManager` in the ctor with
-`path => new MediaVideoSource(path, <W>, <H>, new FfmpegLocator(), new FfmpegDecodeProcess())`,
-subscribe `OnMediaPreviewBitmapChanged` (set `source.VideoImageSource` for every
-`IsMediaSource` with that `MediaPath`), add the `Source { IsMediaSource: true, MediaPath: not null }`
-case to `ResolveOutputFrame` (`MainViewModel.cs:447`), add `or SourceType.MediaSource`
-to `Source.DisplaySource` (Source.cs:119-121) so media previews route `VideoImageSource`,
-and dispose the manager on shutdown. Then slices 2 (native-FPS pacing) and 3 (loop).
+Slice 1 step 4 (wire-in) shipped: `_mediaManager` wired in the core ctor
+(`path => new MediaVideoSource(path, 1920, 1080, new FfmpegLocator(), new FfmpegDecodeProcess())`,
+dispatched preview), `OnMediaPreviewBitmapChanged` sets `VideoImageSource` on every
+`IsMediaSource` with a matching `MediaPath`, a `Source { IsMediaSource, MediaPath }`
+case in `ResolveOutputFrame` feeds `_mediaManager.GetLatestFrame`, `Source.DisplaySource`
+routes `VideoImageSource` for `MediaSource`, and the manager is disposed on shutdown.
+Lives in the new `ViewModels/MainViewModel.Media.cs` partial. **Verified: 14/14
+media + DisplaySource tests pass, build 0 warnings.**
+
+Next (slice 1 step 4's remaining open end): no session acquisition yet — nothing
+`AcquireAsync`s a media path, so no frames flow until the UI picker (a later slice)
+starts a session on add and releases on remove.
+
+Next (slice 2): native-FPS pacing (ffprobe FPS probe + per-frame pacing seam).
+Slice 3: loop control (restart decode on `Completed` when `MediaIsLooping`).
 
 **Landmine:** a stale `testhost` can lock `ytLive.Tests`'s `ytLive.dll` and break
 `dotnet build` of the test project — kill it first (`cmd.exe /c "taskkill /PID N /F"`)
@@ -50,11 +58,11 @@ pure tests run in WSL; user verifies the GUI suites on native Windows PowerShell
 
 ## Next
 
-1. Commit slice 1 step 3 (this unit): manager + `IMediaFrameSource` + tests. Commit
-   is already covered by `TASKS.md`/`HANDOFF.md`; this HANDOFF is the shipping doc.
-2. Slice 1 step 4: wire `_mediaManager` into the resolver + preview routing (see above).
-3. Slice 2: native-FPS pacing (ffprobe FPS probe + per-frame pacing seam).
-4. Await user push checkpoint for the pending TASK 22/21 commits.
+1. Commit slice 1 step 4 (this unit): resolver/preview wire-in + `Source.DisplaySource` media branch + tests.
+2. Slice 2: native-FPS pacing (ffprobe FPS probe + per-frame pacing seam).
+3. Slice 3: loop control (restart decode on `Completed` when `MediaIsLooping`).
+4. UI picker slice: acquire/release media sessions on add/remove (the remaining slice-1-step-4 open end).
+5. Await user push checkpoint for the pending TASK 22/21 commits.
 
 ## The directive (2026-08-31, user)
 

Models/Source.cs

@@ -118,7 +118,7 @@ public class Source : SceneElement
 
     public override ImageSource? DisplaySource => IsLiveCapture
         ? (VideoImageSource ?? _imageSource)
-        : (Type is SourceType.ChatBox or SourceType.WebSource ? VideoImageSource : _imageSource);
+        : (Type is SourceType.ChatBox or SourceType.WebSource or SourceType.MediaSource ? VideoImageSource : _imageSource);
 
     public override bool IsImageSource => Type == SourceType.Image;
 

TASKS.md

@@ -1044,7 +1044,7 @@ The tasks below are ordered by dependency and risk. Each task builds on the prev
 4. ✅ `MediaVideoSource` implements `IMediaFrameSource`: FFmpeg raw video decoder → `VideoFrame` pipeline — spawns ffmpeg `-f rawvideo -pix_fmt bgra`, drains the pipe via pure `RawVideoFrameReader` (`Services/RawVideoFrameReader.cs`), raises `FrameAvailable`/`Completed`; lifecycle is `StartAsync`/`StopAsync`; decode process behind `IDecodeProcess`/`FfmpegDecodeProcess` seam (binary-stdout mirror of `IEncoderProcess`). Shipped 2026-08-31; refactored onto `IMediaFrameSource` with this slice.
 5. ✅ `IMediaFrameSource` + `MediaVideoSourceManager` (`Services/IMediaFrameSource.cs`, `Services/MediaVideoSourceManager.cs`): app-wide decode-session ownership refcounted by `MediaPath` with a `Func<string, IMediaFrameSource?>` factory seam; `AcquireAsync`/`ReleaseAsync`/`ReleaseAllAsync`/`GetLatestFrame`; coalesces frames onto the UI dispatcher onto a single shared `WriteableBitmap` per file (mirror of `ScreenCaptureManager`); `MediaFailed` + `PreviewBitmapChanged`. Unit tests + one integration test (bitmap share/coalesce) — `MediaVideoSourceManagerTests`, 5/5 pass.
 6. 🔶 Native-FPS pacing — frames currently emit as fast as the pipe produces them; ffprobe FPS probe + per-frame pacing seam still open (slice 2).
-7. ☐ Wire into `FramePump` resolver — `Source { Type: MediaSource }` → latest video frame
+7. ☐ Wire into `FramePump` resolver — `Source { Type: MediaSource }` → latest video frame — **resolver + preview routing + manager wired; session acquisition (start/stop on add/remove) still open (comes with the UI picker)**
 8. ☐ Wire into `SceneCompositor` — render media source as an image element at its position/size
 9. ☐ Loop control — `IsLooping` property, restart on end
 10. ☐ Volume control — per-source volume slider for audio playback

ViewModels/MainViewModel.Media.cs

@@ -0,0 +1,35 @@
+using System.Linq;
+using System.Windows.Media.Imaging;
+using ytLive.Models;
+using ytLive.Services;
+
+namespace ytLive.ViewModels;
+
+// Media source (TASK 21): decode sessions are owned app-wide by
+// MediaVideoSourceManager, refcounted by MediaPath. Modeled on the Webcam/Background
+// partials: the readonly manager field is assigned in the core constructor, the
+// shared WriteableBitmap published per file is adopted by every media Source whose
+// MediaPath matches, and ResolveOutputFrame feeds the live compositor.
+public partial class MainViewModel
+{
+    // Decode resolution: the 1920x1080 master. Media frames are scaled to the
+    // element rect by the compositor (UniformToFill), so this is the capture size.
+    private const int MediaDecodeWidth = 1920;
+    private const int MediaDecodeHeight = 1080;
+
+    private readonly MediaVideoSourceManager _mediaManager;
+
+    // MediaVideoSourceManager publishes one shared WriteableBitmap per file; every
+    // media Source pointing at that path picks it up for the preview/canvas.
+    private void OnMediaPreviewBitmapChanged(string path, WriteableBitmap bitmap)
+    {
+        foreach (var source in Scenes.SelectMany(s => s.Elements.OfType<Source>())
+                     .Where(s => s.IsMediaSource && s.MediaPath == path))
+            source.VideoImageSource = bitmap;
+    }
+
+    // Called from the encoder error path is out of scope (TASK 21 sees the decoder
+    // fail at startup); log to the debug console for now, surfaced via the UI later.
+    private static void OnMediaFailed(string path, string message)
+        => System.Diagnostics.Debug.WriteLine($"MediaSource: '{path}' failed: {message}");
+}

ViewModels/MainViewModel.cs

@@ -282,6 +282,14 @@ public partial class MainViewModel : ViewModelBase
         _screenCaptureManager.CaptureFailed += (key, message) =>
             AppLog.Write($"ScreenCaptureManager: capture '{key}' failed: {message}");
 
+        _mediaManager = new MediaVideoSourceManager(
+            path => new MediaVideoSource(
+                path, MediaDecodeWidth, MediaDecodeHeight,
+                new FfmpegLocator(), new FfmpegDecodeProcess()),
+            System.Windows.Application.Current?.Dispatcher);
+        _mediaManager.PreviewBitmapChanged += OnMediaPreviewBitmapChanged;
+        _mediaManager.MediaFailed += OnMediaFailed;
+
         _chatLayer = new ChatOverlayLayer(new ChatBoxRenderer());
 
         _sceneGraph = new SceneGraph();
@@ -397,6 +405,7 @@ public partial class MainViewModel : ViewModelBase
         _framePump.Dispose();
         _cameraManager.Dispose();
         _screenCaptureManager.Dispose();
+        _mediaManager.Dispose();
         _webView2Manager?.Dispose();
         _layoutStore.Dispose();
         _musicPlayer.Dispose();
@@ -452,6 +461,7 @@ public partial class MainViewModel : ViewModelBase
             Source { IsLiveCapture: true, CaptureKey: not null } live => _screenCaptureManager.GetLatestFrame(live.CaptureKey),
             Source { Type: SourceType.ChatBox } chatBox => RenderChatBox(chatBox),
             Source { Type: SourceType.WebSource } web => _webView2Manager?.GetLatestFrame(web.Id),
+            Source { IsMediaSource: true, MediaPath: not null } media => _mediaManager.GetLatestFrame(media.MediaPath),
             Source { AssetId: not null } image => StaticPixelCache.Get(image.AssetId),
             _ => null,
         };

ai.md

@@ -680,8 +680,10 @@ seam:** `Func<Scene?>`, `Func<SceneElement, VideoFrame?>` resolver, `Func<Compos
   (`Services/MediaVideoSourceManager.cs`): refcounted by `MediaPath`, `Func<string, IMediaFrameSource?>` factory
   seam (mirror of `ScreenCaptureManager`), `AcquireAsync`/`ReleaseAsync`/`ReleaseAllAsync`/`GetLatestFrame`,
   coalescing each file's frames onto the UI dispatcher onto one shared `WriteableBitmap`. Frames emit as fast as
-  the pipe produces them; native-FPS pacing (ffprobe probe) and the resolver/preview wire-in (`ResolveOutputFrame`
-  case + `Source.DisplaySource`) are the next slices.
+  the pipe produces them. Wired into the live pipeline (slice 1 step 4): `ResolveOutputFrame` reads
+  `_mediaManager.GetLatestFrame(MediaPath)` for a `MediaSource`, and `Source.DisplaySource`
+  (`Models/Source.cs`) routes `VideoImageSource` for `MediaSource` so the preview/canvas show decoded video.
+  Still open: session acquisition on add/remove (UI picker), native-FPS pacing (ffprobe probe), loop control.
 - **Social bar on the output (bar bug-fix branch):** the `FramePump` takes an optional
   `socialBar: Func<(VideoFrame? Frame, SocialBarPosition Position)>?` seam, re-read **every frame** (so a
   mid-stream position flip applies immediately). The strip is pre-rasterized by `Compositor/SocialBarRenderer.cs`

ytLive.Tests/BackgroundTests.cs

@@ -93,6 +93,20 @@ public class BackgroundTests
         Assert.Same(bitmap, source.DisplaySource);
     }
 
+    // TASK 21: a media source's DisplaySource routes its live video frames (like
+    // Display/Window capture), so media previews show decoded video, not the stale
+    // default art.
+    [Fact]
+    public void Source_DisplaySource_IsVideoImageSource_WhenMediaSource()
+    {
+        var source = new Source { Type = SourceType.MediaSource };
+        var bitmap = new System.Windows.Media.Imaging.WriteableBitmap(
+            2, 2, 96, 96, System.Windows.Media.PixelFormats.Bgra32, null);
+        source.VideoImageSource = bitmap;
+
+        Assert.Same(bitmap, source.DisplaySource);
+    }
+
     [Fact]
     public void Source_DisplaySource_FallsBackToImage_WhenLiveCaptureWithNoVideoImage()
     {