feat(windows): per-app screen-audio sharing (include/exclude)

Adds "only selected apps" and "all apps except selected" screen-audio
modes to the Windows client, alongside the existing entire-desktop path.

Per-app capture uses WASAPI process loopback (AUDCLNT_ACTIVATIONTYPE_
PROCESS_LOOPBACK) via ProcessLoopbackCapture, mixed by ProcessAudioMixer
and fed to the core through vc_stream_feed_pcm (external_feed=1 so the
core skips its own loopback device).

Init must pass AUDCLNT_STREAMFLAGS_LOOPBACK | EVENTCALLBACK |
AUTOCONVERTPCM; the LOOPBACK flag is what makes the virtual endpoint
deliver rendered audio (without it every buffer is flagged SILENT) and
AUTOCONVERTPCM resamples the app's native format to 48k s16.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

clients/windows/VoiceCat.App/Audio/AudioSessionEnumerator.cs

@@ -0,0 +1,205 @@
+using System.Diagnostics;
+using System.Runtime.InteropServices;
+
+namespace VoiceCat.App.Audio;
+
+// ── Scope types (mirror macOS ScreenAudioScope / ScreenAudioSelection) ────────
+
+public abstract record AppAudioScope;
+public sealed record EntireDesktop : AppAudioScope;
+public sealed record OnlyApps(IReadOnlyList<int> Pids, IReadOnlyList<string> Names) : AppAudioScope;
+public sealed record AllExceptApps(IReadOnlyList<int> Pids, IReadOnlyList<string> Names) : AppAudioScope;
+
+public sealed record AudioAppInfo(int Pid, string DisplayName);
+
+// ── Enumerator ─────────────────────────────────────────────────────────────────
+
+public static class AudioSessionEnumerator
+{
+    // Returns all user-facing apps: visible-window processes (primary, like macOS
+    // SCShareableContent.current) plus any background audio-session-only processes
+    // (e.g. Spotify in mini-player). Excludes VoiceCat itself and system processes.
+    public static IReadOnlyList<AudioAppInfo> GetAudioApps()
+    {
+        var seen   = new HashSet<int>();
+        var result = new List<AudioAppInfo>();
+        int selfPid = Environment.ProcessId;
+
+        // ── 1. Visible-window processes (EnumWindows) ─────────────────────────
+        // Same set macOS ScreenCaptureKit exposes: all apps with at least one
+        // visible top-level window. Shows apps even when not currently producing audio.
+        EnumWindows((hWnd, _) =>
+        {
+            if (!IsWindowVisible(hWnd)) return true;
+            GetWindowThreadProcessId(hWnd, out uint pid);
+            if (pid == 0 || pid == (uint)selfPid || !seen.Add((int)pid)) return true;
+
+            try
+            {
+                var proc = Process.GetProcessById((int)pid);
+                string name = proc.MainWindowTitle.Length > 0
+                    ? $"{proc.ProcessName} — {proc.MainWindowTitle}"
+                    : proc.ProcessName;
+                if (!string.IsNullOrEmpty(proc.ProcessName))
+                    result.Add(new AudioAppInfo((int)pid, name));
+            }
+            catch { /* process exited between EnumWindows and GetProcessById */ }
+
+            return true; // continue enumeration
+        }, IntPtr.Zero);
+
+        // ── 2. Background audio-session processes (WASAPI, supplement) ────────
+        // Catches apps that produce audio but have no visible window (screen reader,
+        // background music player, etc.). Silently skipped if WASAPI is unavailable.
+        AppendAudioSessionApps(seen, selfPid, result);
+
+        return result.OrderBy(a => a.DisplayName, StringComparer.OrdinalIgnoreCase).ToList();
+    }
+
+    // ── EnumWindows P/Invoke ──────────────────────────────────────────────────
+
+    private delegate bool EnumWindowsProc(IntPtr hWnd, IntPtr lParam);
+
+    [DllImport("user32.dll")]
+    private static extern bool EnumWindows(EnumWindowsProc lpEnumFunc, IntPtr lParam);
+
+    [DllImport("user32.dll")]
+    private static extern bool IsWindowVisible(IntPtr hWnd);
+
+    [DllImport("user32.dll")]
+    private static extern uint GetWindowThreadProcessId(IntPtr hWnd, out uint lpdwProcessId);
+
+    // ── WASAPI audio session supplement ──────────────────────────────────────
+
+    private static void AppendAudioSessionApps(HashSet<int> seen, int selfPid,
+                                               List<AudioAppInfo> result)
+    {
+        IMMDeviceEnumerator? enumerator = null;
+        IMMDevice? device    = null;
+        IAudioSessionManager2? manager  = null;
+        IAudioSessionEnumerator? sessions = null;
+
+        try
+        {
+            enumerator = (IMMDeviceEnumerator)Activator.CreateInstance(
+                Type.GetTypeFromCLSID(new Guid("BCDE0395-E52F-467C-8E3D-C4579291692E"))!)!;
+
+            enumerator.GetDefaultAudioEndpoint(0 /*eRender*/, 1 /*eMultimedia*/, out device);
+
+            var mgr2Iid = new Guid("77AA99A0-1BD6-484F-8BC7-2C654C9A9B6F");
+            device.Activate(ref mgr2Iid, 0x17 /*CLSCTX_ALL*/, IntPtr.Zero, out object mgr);
+            manager = (IAudioSessionManager2)mgr;
+
+            manager.GetSessionEnumerator(out sessions);
+            sessions.GetCount(out int count);
+
+            for (int i = 0; i < count; i++)
+            {
+                IAudioSessionControl? ctrl = null;
+                try
+                {
+                    sessions.GetSession(i, out ctrl);
+                    var ctrl2 = (IAudioSessionControl2)ctrl;
+                    ctrl2.GetProcessId(out uint pid);
+
+                    int ipid = (int)pid;
+                    if (pid == 0 || ipid == selfPid || !seen.Add(ipid)) continue;
+
+                    try
+                    {
+                        var proc = Process.GetProcessById(ipid);
+                        if (!string.IsNullOrEmpty(proc.ProcessName))
+                            result.Add(new AudioAppInfo(ipid, proc.ProcessName));
+                    }
+                    catch { /* exited */ }
+                }
+                catch { /* stale session */ }
+                finally { if (ctrl != null) Marshal.ReleaseComObject(ctrl); }
+            }
+        }
+        catch { /* no audio device or WASAPI unavailable — ignore */ }
+        finally
+        {
+            if (sessions  != null) Marshal.ReleaseComObject(sessions);
+            if (manager   != null) Marshal.ReleaseComObject(manager);
+            if (device    != null) Marshal.ReleaseComObject(device);
+            if (enumerator != null) Marshal.ReleaseComObject(enumerator);
+        }
+    }
+
+    // ── COM interface declarations ─────────────────────────────────────────────
+
+    [ComImport, Guid("A95664D2-9614-4F35-A746-DE8DB63617E6"),
+     InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
+    internal interface IMMDeviceEnumerator
+    {
+        [PreserveSig] int EnumAudioEndpoints(int dataFlow, int stateMask, out IntPtr devices);
+        [PreserveSig] int GetDefaultAudioEndpoint(int dataFlow, int role, out IMMDevice endpoint);
+        [PreserveSig] int GetDevice([MarshalAs(UnmanagedType.LPWStr)] string id, out IMMDevice device);
+        [PreserveSig] int RegisterEndpointNotificationCallback(IntPtr client);
+        [PreserveSig] int UnregisterEndpointNotificationCallback(IntPtr client);
+    }
+
+    [ComImport, Guid("D666063F-1587-4E43-81F1-B948E807363F"),
+     InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
+    internal interface IMMDevice
+    {
+        [PreserveSig] int Activate(ref Guid iid, int clsCtx, IntPtr activationParams,
+            [MarshalAs(UnmanagedType.IUnknown)] out object ppInterface);
+        [PreserveSig] int OpenPropertyStore(int stgmAccess, out IntPtr propStore);
+        [PreserveSig] int GetId([MarshalAs(UnmanagedType.LPWStr)] out string id);
+        [PreserveSig] int GetState(out int state);
+    }
+
+    [ComImport, Guid("77AA99A0-1BD6-484F-8BC7-2C654C9A9B6F"),
+     InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
+    internal interface IAudioSessionManager2
+    {
+        [PreserveSig] int GetAudioSessionControl(ref Guid audioSessionGuid, int streamFlags,
+            out IAudioSessionControl session);
+        [PreserveSig] int GetSimpleAudioVolume(ref Guid audioSessionGuid, int streamFlags,
+            out IntPtr audioVolume);
+        [PreserveSig] int GetSessionEnumerator(out IAudioSessionEnumerator sessionEnum);
+        [PreserveSig] int RegisterSessionNotification(IntPtr notification);
+        [PreserveSig] int UnregisterSessionNotification(IntPtr notification);
+        [PreserveSig] int RegisterDuckNotification([MarshalAs(UnmanagedType.LPWStr)] string sessionID,
+            IntPtr notification);
+        [PreserveSig] int UnregisterDuckNotification(IntPtr notification);
+    }
+
+    [ComImport, Guid("E2F5BB11-0570-40CA-ACDD-3AA01277DEE8"),
+     InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
+    internal interface IAudioSessionEnumerator
+    {
+        [PreserveSig] int GetCount(out int sessionCount);
+        [PreserveSig] int GetSession(int sessionIndex, out IAudioSessionControl session);
+    }
+
+    [ComImport, Guid("F4B1A599-7266-4319-A8CA-E70ACB11E8CD"),
+     InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
+    internal interface IAudioSessionControl
+    {
+        [PreserveSig] int GetState(out int state);
+        [PreserveSig] int GetDisplayName([MarshalAs(UnmanagedType.LPWStr)] out string name);
+        [PreserveSig] int SetDisplayName([MarshalAs(UnmanagedType.LPWStr)] string name,
+            ref Guid eventContext);
+        [PreserveSig] int GetIconPath([MarshalAs(UnmanagedType.LPWStr)] out string iconPath);
+        [PreserveSig] int SetIconPath([MarshalAs(UnmanagedType.LPWStr)] string iconPath,
+            ref Guid eventContext);
+        [PreserveSig] int GetGroupingParam(out Guid groupingParam);
+        [PreserveSig] int SetGroupingParam(ref Guid groupingParam, ref Guid eventContext);
+        [PreserveSig] int RegisterAudioSessionNotification(IntPtr notification);
+        [PreserveSig] int UnregisterAudioSessionNotification(IntPtr notification);
+    }
+
+    [ComImport, Guid("BFB7FF88-7239-4FC9-8FA2-07C950BE9C6D"),
+     InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
+    internal interface IAudioSessionControl2 : IAudioSessionControl
+    {
+        [PreserveSig] int GetSessionIdentifier([MarshalAs(UnmanagedType.LPWStr)] out string id);
+        [PreserveSig] int GetSessionInstanceIdentifier([MarshalAs(UnmanagedType.LPWStr)] out string id);
+        [PreserveSig] int GetProcessId(out uint pid);
+        [PreserveSig] int IsSystemSoundsSession();
+        [PreserveSig] int SetDuckingPreference(bool optOut);
+    }
+}

clients/windows/VoiceCat.App/Audio/ProcessAudioMixer.cs

@@ -0,0 +1,146 @@
+using VoiceCat.Interop;
+
+// Owns N ProcessLoopbackCapture instances, mixes their PCM every 20 ms, and feeds
+// the result to the core via vc_stream_feed_pcm. Used for per-app audio sharing.
+namespace VoiceCat.App.Audio;
+
+public sealed class ProcessAudioMixer : IDisposable
+{
+    private const int SampleRate   = 48000;
+    private const int FrameSamples = 960;
+    private const int Channels     = 2; // stereo; captures fall back to mono if needed
+
+    private readonly List<ProcessLoopbackCapture> _captures = [];
+
+    // Per-capture latest frame, protected by _frameLock.
+    private readonly object _frameLock = new();
+    private List<short[]>   _latestFrames = [];
+    private int             _activeChannels = Channels;
+
+    private Thread?          _mixThread;
+    private volatile bool    _running;
+    private VoiceCatClient?  _client;
+    private uint             _streamId;
+
+    public void Start(AppAudioScope scope, VoiceCatClient client, uint streamId,
+                      IReadOnlyList<AudioAppInfo> allApps)
+    {
+        if (_running) return;
+        _client   = client;
+        _streamId = streamId;
+
+        var pids = ResolvePids(scope, allApps);
+        if (pids.Count == 0)
+        {
+            // nothing to capture — scope resolved to empty set
+            return;
+        }
+
+        lock (_frameLock)
+        {
+            _latestFrames   = new List<short[]>(new short[pids.Count][]);
+            _activeChannels = Channels;
+        }
+
+        for (int i = 0; i < pids.Count; i++)
+        {
+            int captureIndex = i;
+            var cap = new ProcessLoopbackCapture(pids[i], ProcessLoopbackCapture.Mode.Include);
+            cap.PcmFrameReady += (pcm, spc, ch) => OnCaptureFrame(captureIndex, pcm, ch);
+            _captures.Add(cap);
+        }
+
+        foreach (var c in _captures) c.Start();
+
+        _running   = true;
+        _mixThread = new Thread(MixLoop) { IsBackground = true, Name = "ProcessAudioMixer" };
+        _mixThread.Start();
+    }
+
+    public void Stop()
+    {
+        _running = false;
+        _mixThread?.Join(500);
+        foreach (var c in _captures) { c.Stop(); c.Dispose(); }
+        _captures.Clear();
+    }
+
+    public void Dispose() => Stop();
+
+    // ── Capture callback ──────────────────────────────────────────────────────
+
+    private void OnCaptureFrame(int index, short[] pcm, int channels)
+    {
+        lock (_frameLock)
+        {
+            // Upmix mono → stereo interleave if the capture fell back to mono.
+            if (channels == 1 && _activeChannels == 2)
+                pcm = MonoToStereo(pcm);
+            if (index < _latestFrames.Count)
+                _latestFrames[index] = pcm;
+        }
+    }
+
+    // ── Mix loop (20 ms timer) ────────────────────────────────────────────────
+
+    private void MixLoop()
+    {
+        // Use a target period close to 20 ms; small under-shoot avoids accumulating drift.
+        const int periodMs = 19;
+        while (_running)
+        {
+            Thread.Sleep(periodMs);
+            if (!_running) break;
+
+            short[] mix;
+            lock (_frameLock)
+            {
+                int len = FrameSamples * _activeChannels;
+                mix = new short[len];
+
+                foreach (var frame in _latestFrames)
+                {
+                    if (frame == null) continue;
+                    int frameLen = Math.Min(frame.Length, len);
+                    for (int i = 0; i < frameLen; i++)
+                    {
+                        int sum = mix[i] + frame[i];
+                        // Saturating clamp
+                        mix[i] = (short)Math.Clamp(sum, short.MinValue, short.MaxValue);
+                    }
+                }
+            }
+
+            _client?.StreamFeedPcm(_streamId, mix, FrameSamples, (uint)_activeChannels);
+        }
+    }
+
+    // ── Helpers ───────────────────────────────────────────────────────────────
+
+    // For "AllExcept": enumerate all running audio apps and exclude the specified ones.
+    // For "OnlyApps": use their PIDs directly.
+    private static List<int> ResolvePids(AppAudioScope scope, IReadOnlyList<AudioAppInfo> allApps)
+    {
+        return scope switch
+        {
+            OnlyApps    o => [.. o.Pids],
+            AllExceptApps a =>
+                allApps
+                    .Where(app => !a.Pids.Contains(app.Pid))
+                    .Select(app => app.Pid)
+                    .ToList(),
+            _ => [],
+        };
+    }
+
+    private static short[] MonoToStereo(short[] mono)
+    {
+        var stereo = new short[mono.Length * 2];
+        for (int i = 0; i < mono.Length; i++)
+        {
+            stereo[i * 2]     = mono[i];
+            stereo[i * 2 + 1] = mono[i];
+        }
+        return stereo;
+    }
+}

clients/windows/VoiceCat.App/Audio/ProcessLoopbackCapture.cs

@@ -0,0 +1,434 @@
+using System.Runtime.InteropServices;
+
+// Single-process WASAPI loopback capture via AUDIOCLIENT_ACTIVATION_PARAMS
+// (Windows 10 2004+ / Build 19041+).
+//
+// Threading: ALL WASAPI init runs on the capture thread (MTA). If called from the
+// WinForms UI thread (STA), ActivateAudioInterfaceAsync fires ActivateCompleted on
+// an MTA pool thread; COM marshals that back to the STA pump — but the STA thread is
+// blocked on CompletionEvent.Wait → deadlock. MTA capture thread avoids this.
+//
+// COM QI policy: the COM objects returned by the process-loopback activation path
+// reject QueryInterface for their own IIDs under .NET's RCW mechanism. Every call
+// to IAudioClient and IAudioCaptureClient is therefore dispatched via raw vtable
+// pointer arithmetic, bypassing .NET COM interop entirely.
+namespace VoiceCat.App.Audio;
+
+public sealed class ProcessLoopbackCapture : IDisposable
+{
+    public enum Mode { Include, Exclude }
+
+    // Fired on the capture thread every 20 ms (960 samples/channel @ 48 kHz, interleaved s16).
+    public event Action<short[], int /*samplesPerChannel*/, int /*channels*/>? PcmFrameReady;
+
+    private const int SampleRate   = 48000;
+    private const int FrameSamples = 960;  // 20 ms
+    private const string LoopbackDevicePath = "VAD\\Process_Loopback";
+
+    private readonly int  _pid;
+    private readonly Mode _mode;
+
+    // Raw COM pointers — managed via explicit AddRef/Release, no RCW wrapping.
+    private IntPtr _audioClientPtr;   // IAudioClient*
+    private IntPtr _captureClientPtr; // IAudioCaptureClient*
+
+    private AutoResetEvent? _bufferEvent;
+    private Thread?         _captureThread;
+    private volatile bool   _running;
+    private int             _channels;
+
+    // Accumulator: assembles driver-callback-sized fragments into FrameSamples chunks.
+    private short[] _accumBuf = [];
+    private int     _accumCount;
+
+    // Init-done signal: Set() by the capture thread after ActivateClient() completes.
+    private readonly ManualResetEventSlim _initDone = new(false);
+    private bool _initOk;
+
+    public ProcessLoopbackCapture(int pid, Mode mode)
+    {
+        _pid  = pid;
+        _mode = mode;
+    }
+
+    /// <summary>Starts capture. Blocks until WASAPI activation completes (typically &lt;100 ms).
+    /// Returns false if the process cannot be captured.</summary>
+    public bool Start()
+    {
+        if (_running) return false;
+        _running       = true;
+        _captureThread = new Thread(CaptureThreadProc)
+        {
+            IsBackground = true,
+            Name         = $"ProcLoopback:{_pid}",
+        };
+        _captureThread.Start();
+
+        bool ok = _initDone.Wait(5000) && _initOk;
+        if (!ok) _running = false;
+        return ok;
+    }
+
+    public void Stop()
+    {
+        _running = false;
+        _bufferEvent?.Set();
+        _captureThread?.Join(500);
+        if (_audioClientPtr != IntPtr.Zero) AC_Stop(_audioClientPtr);
+    }
+
+    public void Dispose()
+    {
+        Stop();
+        ComRelease(ref _captureClientPtr);
+        ComRelease(ref _audioClientPtr);
+        _bufferEvent?.Dispose();
+        _initDone.Dispose();
+    }
+
+    // ── Capture thread (MTA) ──────────────────────────────────────────────────
+
+    private void CaptureThreadProc()
+    {
+        _initOk = ActivateAndStart();
+        _initDone.Set();
+        if (!_initOk) return;
+        CaptureLoop();
+    }
+
+    private bool ActivateAndStart()
+    {
+        if (!ActivateClient()) return false;
+
+        _bufferEvent = new AutoResetEvent(false);
+        if (AC_SetEventHandle(_audioClientPtr, _bufferEvent.SafeWaitHandle.DangerousGetHandle()) < 0)
+            return false;
+
+        return AC_Start(_audioClientPtr) >= 0;
+    }
+
+    // ── Activation ───────────────────────────────────────────────────────────
+
+    private unsafe bool ActivateClient()
+    {
+        var activationParams = new AudioClientActivationParams
+        {
+            ActivationType      = 1,   // AUDCLNT_ACTIVATIONTYPE_PROCESS_LOOPBACK
+            TargetProcessId     = (uint)_pid,
+            ProcessLoopbackMode = _mode == Mode.Include ? 0u : 1u,
+        };
+
+        var handler = new ActivationCompletionHandler();
+        var audioClientIid = new Guid("1CB9AD4C-DBFA-4c32-B178-C2F568A703B2");
+
+        IntPtr opPtr;
+        {
+            AudioClientActivationParams* pParams = &activationParams;
+            // PROPVARIANT (VT_BLOB) x64: vt(2)+res(6)+cbSize(4)+pad(4)+pBlobData(8) = 24 B.
+            var pv = stackalloc byte[24];
+            *(ushort*)(pv + 0)  = 65;
+            *(uint*)  (pv + 8)  = (uint)sizeof(AudioClientActivationParams);
+            *(nint*)  (pv + 16) = (nint)pParams;
+
+            int hr = ActivateAudioInterfaceAsync(
+                LoopbackDevicePath, ref audioClientIid,
+                (IntPtr)pv, handler, out opPtr);
+            if (hr < 0) return false;
+        }
+
+        if (!handler.CompletionEvent.Wait(3000))
+        {
+            ComRelease(ref opPtr);
+            return false;
+        }
+
+        // Call GetActivateResult via vtable (slot 3) — avoids QI on the async-op object.
+        if (!Vtable_GetActivateResult(handler.OperationPtr, out int activateHr, out IntPtr activatedPtr))
+        {
+            handler.ReleaseOp();
+            ComRelease(ref opPtr);
+            return false;
+        }
+        handler.ReleaseOp();
+        ComRelease(ref opPtr);
+
+        if (activateHr < 0 || activatedPtr == IntPtr.Zero) return false;
+
+        // Store the raw IAudioClient* — do NOT create an RCW; use vtable dispatch instead.
+        _audioClientPtr = activatedPtr;
+        // activatedPtr already has ref count from GetActivateResult; don't double-release.
+
+        return InitializeStream();
+    }
+
+    // IActivateAudioInterfaceAsyncOperation vtable slot 3: GetActivateResult(HRESULT*, IUnknown**)
+    private static unsafe bool Vtable_GetActivateResult(IntPtr op,
+        out int activateHr, out IntPtr activatedPtr)
+    {
+        activateHr   = unchecked((int)0x80004005);
+        activatedPtr = IntPtr.Zero;
+        if (op == IntPtr.Zero) return false;
+        void** vtable = *(void***)op.ToPointer();
+        var fn = (delegate* unmanaged[Stdcall]<IntPtr, int*, IntPtr*, int>)vtable[3];
+        fixed (int*    pHr  = &activateHr)
+        fixed (IntPtr* pPtr = &activatedPtr)
+            return fn(op, pHr, pPtr) >= 0;
+    }
+
+    private unsafe bool InitializeStream()
+    {
+        // Try s16 stereo first; fall back to s16 mono.
+        foreach (int ch in new[] { 2, 1 })
+        {
+            var fmt = new WaveFormatEx
+            {
+                wFormatTag      = 1,           // WAVE_FORMAT_PCM
+                nChannels       = (ushort)ch,
+                nSamplesPerSec  = SampleRate,
+                wBitsPerSample  = 16,
+                nBlockAlign     = (ushort)(ch * 2),
+                nAvgBytesPerSec = (uint)(SampleRate * ch * 2),
+                cbSize          = 0,
+            };
+
+            // Process-loopback requires LOOPBACK (deliver rendered audio) + EVENTCALLBACK +
+            // AUTOCONVERTPCM (resample the app's native format to our requested s16/48k). Without
+            // LOOPBACK every buffer comes back AUDCLNT_BUFFERFLAGS_SILENT; without AUTOCONVERTPCM
+            // the requested format is rejected. Matches the MS ApplicationLoopback sample.
+            //   AUDCLNT_SHAREMODE_SHARED      = 0
+            //   AUDCLNT_STREAMFLAGS_LOOPBACK  = 0x00020000
+            //   AUDCLNT_STREAMFLAGS_EVENTCALLBACK = 0x00040000
+            //   AUDCLNT_STREAMFLAGS_AUTOCONVERTPCM = 0x80000000
+            const uint streamFlags = 0x00020000u | 0x00040000u | 0x80000000u;
+            int hr = AC_Initialize(_audioClientPtr, 0, streamFlags,
+                                   2_000_000 /*200ms hns*/, 0, &fmt, null);
+            if (hr >= 0)
+            {
+                _channels   = ch;
+                _accumBuf   = new short[FrameSamples * ch];
+                _accumCount = 0;
+                break;
+            }
+            if (ch == 1) return false;
+        }
+
+        var captureIid = new Guid("C8ADBD64-E71E-48a0-A4DE-185C395CD317");
+        int getHr = AC_GetService(_audioClientPtr, ref captureIid, out _captureClientPtr);
+        return getHr >= 0 && _captureClientPtr != IntPtr.Zero;
+    }
+
+    // ── Capture loop ─────────────────────────────────────────────────────────
+
+    private void CaptureLoop()
+    {
+        while (_running)
+        {
+            _bufferEvent!.WaitOne(100);
+            if (!_running) break;
+
+            while (_running)
+            {
+                int hr = CC_GetNextPacketSize(_captureClientPtr, out uint packetSize);
+                if (hr < 0 || packetSize == 0) break;
+
+                hr = CC_GetBuffer(_captureClientPtr, out IntPtr dataPtr, out uint framesAvailable,
+                    out uint flags);
+                if (hr < 0) break;
+
+                bool silent = (flags & 2) != 0; // AUDCLNT_BUFFERFLAGS_SILENT
+                if (framesAvailable > 0)
+                {
+                    if (silent) AccumulateSilence((int)framesAvailable);
+                    else        AccumulatePcm(dataPtr, (int)framesAvailable);
+                }
+
+                CC_ReleaseBuffer(_captureClientPtr, framesAvailable);
+            }
+        }
+    }
+
+    private unsafe void AccumulatePcm(IntPtr data, int frames)
+    {
+        var src   = (short*)data.ToPointer();
+        int total = frames * _channels;
+        int idx   = 0;
+        while (idx < total)
+        {
+            int space = _accumBuf.Length - _accumCount;
+            int copy  = Math.Min(total - idx, space);
+            fixed (short* dst = _accumBuf)
+                Buffer.MemoryCopy(src + idx, dst + _accumCount, copy * 2L, copy * 2L);
+            _accumCount += copy;
+            idx         += copy;
+            if (_accumCount == _accumBuf.Length)
+                FlushFrame();
+        }
+    }
+
+    private void AccumulateSilence(int frames)
+    {
+        int total = frames * _channels;
+        int idx   = 0;
+        while (idx < total)
+        {
+            int space = _accumBuf.Length - _accumCount;
+            int fill  = Math.Min(total - idx, space);
+            Array.Clear(_accumBuf, _accumCount, fill);
+            _accumCount += fill;
+            idx         += fill;
+            if (_accumCount == _accumBuf.Length)
+                FlushFrame();
+        }
+    }
+
+    private void FlushFrame()
+    {
+        var copy = new short[_accumBuf.Length];
+        _accumBuf.AsSpan().CopyTo(copy);
+        PcmFrameReady?.Invoke(copy, FrameSamples, _channels);
+        _accumCount = 0;
+    }
+
+    // ── IAudioClient vtable helpers (raw dispatch, no RCW / no QI) ───────────
+    //
+    // Vtable layout (IUnknown base: 0=QI 1=AddRef 2=Release; then IAudioClient methods):
+    //   3=Initialize  4=GetBufferSize  5=GetStreamLatency  6=GetCurrentPadding
+    //   7=IsFormatSupported  8=GetMixFormat  9=GetDevicePeriod
+    //   10=Start  11=Stop  12=Reset  13=SetEventHandle  14=GetService
+
+    private static unsafe int AC_Initialize(IntPtr ac, int shareMode, uint streamFlags,
+        long hnsBufferDuration, long hnsPeriodicity, WaveFormatEx* pFormat, Guid* pSession)
+    {
+        var fn = (delegate* unmanaged[Stdcall]<IntPtr, int, uint, long, long, WaveFormatEx*, Guid*, int>)
+            (*(void***)ac)[3];
+        return fn(ac, shareMode, streamFlags, hnsBufferDuration, hnsPeriodicity, pFormat, pSession);
+    }
+
+    private static unsafe int AC_Start(IntPtr ac)
+    {
+        var fn = (delegate* unmanaged[Stdcall]<IntPtr, int>)(*(void***)ac)[10];
+        return fn(ac);
+    }
+
+    private static unsafe int AC_Stop(IntPtr ac)
+    {
+        var fn = (delegate* unmanaged[Stdcall]<IntPtr, int>)(*(void***)ac)[11];
+        return fn(ac);
+    }
+
+    private static unsafe int AC_SetEventHandle(IntPtr ac, IntPtr eventHandle)
+    {
+        var fn = (delegate* unmanaged[Stdcall]<IntPtr, IntPtr, int>)(*(void***)ac)[13];
+        return fn(ac, eventHandle);
+    }
+
+    private static unsafe int AC_GetService(IntPtr ac, ref Guid riid, out IntPtr ppv)
+    {
+        var fn = (delegate* unmanaged[Stdcall]<IntPtr, Guid*, IntPtr*, int>)(*(void***)ac)[14];
+        fixed (Guid* pIid = &riid)
+        fixed (IntPtr* pPpv = &ppv)
+            return fn(ac, pIid, pPpv);
+    }
+
+    // ── IAudioCaptureClient vtable helpers ────────────────────────────────────
+    //
+    // Vtable (IUnknown: 0-2; then): 3=GetBuffer  4=ReleaseBuffer  5=GetNextPacketSize
+
+    private static unsafe int CC_GetBuffer(IntPtr cc, out IntPtr ppData,
+        out uint pNumFrames, out uint pdwFlags)
+    {
+        var fn = (delegate* unmanaged[Stdcall]<IntPtr, IntPtr*, uint*, uint*, ulong*, ulong*, int>)
+            (*(void***)cc)[3];
+        ulong devPos = 0, qpcPos = 0;
+        fixed (IntPtr* p0 = &ppData)
+        fixed (uint*   p1 = &pNumFrames)
+        fixed (uint*   p2 = &pdwFlags)
+            return fn(cc, p0, p1, p2, &devPos, &qpcPos);
+    }
+
+    private static unsafe int CC_ReleaseBuffer(IntPtr cc, uint numFrames)
+    {
+        var fn = (delegate* unmanaged[Stdcall]<IntPtr, uint, int>)(*(void***)cc)[4];
+        return fn(cc, numFrames);
+    }
+
+    private static unsafe int CC_GetNextPacketSize(IntPtr cc, out uint pNumFrames)
+    {
+        var fn = (delegate* unmanaged[Stdcall]<IntPtr, uint*, int>)(*(void***)cc)[5];
+        fixed (uint* p = &pNumFrames)
+            return fn(cc, p);
+    }
+
+    // ── COM utilities ─────────────────────────────────────────────────────────
+
+    private static unsafe void ComRelease(ref IntPtr ptr)
+    {
+        if (ptr == IntPtr.Zero) return;
+        var fn = (delegate* unmanaged[Stdcall]<IntPtr, uint>)(*(void***)ptr)[2]; // IUnknown::Release
+        fn(ptr);
+        ptr = IntPtr.Zero;
+    }
+
+    // ── P/Invoke & structs ────────────────────────────────────────────────────
+
+    [DllImport("Mmdevapi.dll", CharSet = CharSet.Unicode)]
+    private static extern int ActivateAudioInterfaceAsync(
+        string deviceInterfacePath,
+        ref Guid riid,
+        IntPtr activationParams,
+        [MarshalAs(UnmanagedType.Interface)] IActivateAudioInterfaceCompletionHandler completionHandler,
+        out IntPtr activationOperation);
+
+    [StructLayout(LayoutKind.Sequential)]
+    private struct AudioClientActivationParams
+    {
+        public int  ActivationType;       // AUDCLNT_ACTIVATIONTYPE_PROCESS_LOOPBACK = 1
+        public uint TargetProcessId;
+        public uint ProcessLoopbackMode;  // INCLUDE=0, EXCLUDE=1
+    }
+
+    [StructLayout(LayoutKind.Sequential, Pack = 2)]
+    private struct WaveFormatEx
+    {
+        public ushort wFormatTag;
+        public ushort nChannels;
+        public uint   nSamplesPerSec;
+        public uint   nAvgBytesPerSec;
+        public ushort nBlockAlign;
+        public ushort wBitsPerSample;
+        public ushort cbSize;
+    }
+
+    // ── Completion handler CCW ────────────────────────────────────────────────
+    //
+    // Only this object still uses .NET COM interop (as a CCW). The activateOperation
+    // parameter is IntPtr to avoid QI on the incoming async-op pointer.
+
+    [ComVisible(true), ClassInterface(ClassInterfaceType.None)]
+    private sealed class ActivationCompletionHandler : IActivateAudioInterfaceCompletionHandler
+    {
+        public readonly ManualResetEventSlim CompletionEvent = new(false);
+        public IntPtr OperationPtr { get; private set; }
+
+        public void ActivateCompleted(IntPtr activateOperation)
+        {
+            OperationPtr = activateOperation;
+            if (OperationPtr != IntPtr.Zero) Marshal.AddRef(OperationPtr);
+            CompletionEvent.Set();
+        }
+
+        public void ReleaseOp()
+        {
+            if (OperationPtr == IntPtr.Zero) return;
+            Marshal.Release(OperationPtr);
+            OperationPtr = IntPtr.Zero;
+        }
+    }
+
+    [ComImport, Guid("41D949AB-9862-444A-80F6-C261334DA5EB"),
+     InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
+    private interface IActivateAudioInterfaceCompletionHandler
+    {
+        void ActivateCompleted(IntPtr activateOperation);
+    }
+}

clients/windows/VoiceCat.App/Forms/AppAudioPickerDialog.cs

@@ -0,0 +1,166 @@
+using VoiceCat.App.Audio;
+
+namespace VoiceCat.App.Forms;
+
+/// <summary>
+/// Modal dialog for selecting which apps' audio to share.
+/// Returns the chosen <see cref="AppAudioScope"/> via <see cref="ChosenScope"/>,
+/// or <see cref="DialogResult.Cancel"/> if the user dismissed without sharing.
+/// </summary>
+public sealed class AppAudioPickerDialog : Form
+{
+    private readonly RadioButton  _rdoAll;
+    private readonly RadioButton  _rdoOnly;
+    private readonly RadioButton  _rdoExcept;
+    private readonly ListView     _appList;
+    private readonly Label        _lblApps;
+
+    // Snapshot taken when the dialog opens (refresh on open, not on every check change).
+    private IReadOnlyList<AudioAppInfo> _apps = [];
+
+    public AppAudioScope? ChosenScope { get; private set; }
+
+    public AppAudioPickerDialog()
+    {
+        // ── Radio buttons ───────────────────────────────────────────────────
+        _rdoAll = new RadioButton
+        {
+            Text      = "&Entire desktop",
+            Checked   = true,
+            Location  = new Point(12, 12),
+            Size      = new Size(360, 20),
+            TabIndex  = 0,
+        };
+        _rdoOnly = new RadioButton
+        {
+            Text     = "&Only selected apps",
+            Location = new Point(12, 36),
+            Size     = new Size(360, 20),
+            TabIndex = 1,
+        };
+        _rdoExcept = new RadioButton
+        {
+            Text     = "All apps e&xcept selected",
+            Location = new Point(12, 60),
+            Size     = new Size(360, 20),
+            TabIndex = 2,
+        };
+
+        _rdoAll.CheckedChanged    += OnModeChanged;
+        _rdoOnly.CheckedChanged   += OnModeChanged;
+        _rdoExcept.CheckedChanged += OnModeChanged;
+
+        // ── App list ────────────────────────────────────────────────────────
+        _lblApps = new Label
+        {
+            Text     = "Apps with active audio sessions:",
+            AutoSize = true,
+            Location = new Point(12, 90),
+            Visible  = false,
+            TabIndex = 3,
+        };
+
+        _appList = new ListView
+        {
+            Location      = new Point(12, 112),
+            Size          = new Size(360, 180),
+            CheckBoxes    = true,
+            View          = View.List,
+            Visible       = false,
+            TabIndex      = 4,
+            FullRowSelect = true,
+        };
+
+        // ── Buttons ─────────────────────────────────────────────────────────
+        var btnShare = new Button
+        {
+            Text         = "&Share",
+            DialogResult = DialogResult.OK,
+            Location     = new Point(216, 308),
+            Size         = new Size(75, 27),
+            TabIndex     = 5,
+        };
+        var btnCancel = new Button
+        {
+            Text         = "&Cancel",
+            DialogResult = DialogResult.Cancel,
+            Location     = new Point(297, 308),
+            Size         = new Size(75, 27),
+            TabIndex     = 6,
+        };
+        btnShare.Click += OnShareClick;
+
+        AcceptButton  = btnShare;
+        CancelButton  = btnCancel;
+        AutoScaleMode = AutoScaleMode.Font;
+        ClientSize    = new Size(384, 348);
+        Controls.AddRange([_rdoAll, _rdoOnly, _rdoExcept, _lblApps, _appList, btnShare, btnCancel]);
+        FormBorderStyle = FormBorderStyle.FixedDialog;
+        MaximizeBox     = false;
+        MinimizeBox     = false;
+        StartPosition   = FormStartPosition.CenterParent;
+        Text            = "Share App Audio";
+    }
+
+    protected override void OnLoad(EventArgs e)
+    {
+        base.OnLoad(e);
+        RefreshAppList();
+    }
+
+    private void RefreshAppList()
+    {
+        _apps = AudioSessionEnumerator.GetAudioApps();
+        _appList.Items.Clear();
+        foreach (var app in _apps)
+            _appList.Items.Add(new ListViewItem($"{app.DisplayName}  (PID {app.Pid})") { Tag = app.Pid });
+    }
+
+    private void OnModeChanged(object? sender, EventArgs e)
+    {
+        bool showList = _rdoOnly.Checked || _rdoExcept.Checked;
+        _lblApps.Visible = showList;
+        _appList.Visible = showList;
+
+        if (showList && _appList.Items.Count == 0)
+            RefreshAppList();
+    }
+
+    private void OnShareClick(object? sender, EventArgs e)
+    {
+        if (_rdoAll.Checked)
+        {
+            ChosenScope = new EntireDesktop();
+            return;
+        }
+
+        var checkedPids   = new List<int>();
+        var checkedNames  = new List<string>();
+        foreach (ListViewItem item in _appList.CheckedItems)
+        {
+            if (item.Tag is int pid)
+            {
+                checkedPids.Add(pid);
+                checkedNames.Add(item.Text);
+            }
+        }
+
+        if (checkedPids.Count == 0)
+        {
+            MessageBox.Show(
+                "Select at least one app, or choose 'Entire desktop'.",
+                "No apps selected",
+                MessageBoxButtons.OK,
+                MessageBoxIcon.Warning);
+            DialogResult = DialogResult.None; // prevent close
+            return;
+        }
+
+        ChosenScope = _rdoOnly.Checked
+            ? new OnlyApps(checkedPids, checkedNames)
+            : new AllExceptApps(checkedPids, checkedNames);
+    }
+
+    /// <summary>All apps that were visible in the list when the user clicked Share.</summary>
+    public IReadOnlyList<AudioAppInfo> VisibleApps => _apps;
+}

clients/windows/VoiceCat.App/Forms/MainForm.cs

@@ -1,3 +1,4 @@
+using VoiceCat.App.Audio;
 using VoiceCat.Interop;
 
 namespace VoiceCat.App.Forms;
@@ -22,6 +23,7 @@ public partial class MainForm : Form
     // Voice state
     private uint _micStreamId; // 0 = not started
     private uint _screenStreamId; // 0 = not sharing screen audio
+    private ProcessAudioMixer? _screenMixer; // non-null only in per-app capture mode
     private Keys _pttKey = Keys.F8;
     private bool _serverMuted;
     private bool _serverDeafened;
@@ -442,6 +444,7 @@ public partial class MainForm : Form
         _currentChannelId = 0;
         _micStreamId = 0;
         _screenStreamId = 0;
+        _screenMixer?.Stop(); _screenMixer?.Dispose(); _screenMixer = null;
         txtCompose.Enabled = false;
         btnSend.Enabled = false;
         tsbJoinVoice.Enabled = false;
@@ -622,29 +625,73 @@ public partial class MainForm : Form
     {
         if (_screenStreamId == 0)
         {
-            var (result, streamId) = _client.StartStream(VcStreamKind.ScreenAudio, "Desktop audio");
-            if (result == VcResult.Ok)
-            {
-                _screenStreamId = streamId;
-                tsbScreenShare.Text = "Stop Screen Audio";
-                _miScreenShare.Text = "Stop Screen &Audio";
-                AddActivity("Started sharing screen audio");
-            }
-            else
-            {
-                AddActivity($"Failed to start screen audio: {result}");
-            }
+            StartScreenAudio();
         }
         else
         {
-            _client.StopStream(_screenStreamId);
-            _screenStreamId = 0;
-            tsbScreenShare.Text = "Share Screen Audio";
-            _miScreenShare.Text = "Share Screen &Audio";
-            AddActivity("Stopped sharing screen audio");
+            StopScreenAudio();
         }
     }
 
+    private void StartScreenAudio()
+    {
+        using var picker = new AppAudioPickerDialog();
+        if (picker.ShowDialog(this) != DialogResult.OK || picker.ChosenScope == null) return;
+
+        var scope = picker.ChosenScope;
+
+        if (scope is EntireDesktop)
+        {
+            // Existing whole-device WASAPI loopback path — core handles it.
+            var (result, streamId) = _client.StartStream(VcStreamKind.ScreenAudio, "Desktop audio");
+            if (result != VcResult.Ok)
+            {
+                AddActivity($"Failed to start screen audio: {result}");
+                return;
+            }
+            _screenStreamId = streamId;
+            AddActivity("Sharing screen audio: entire desktop");
+        }
+        else
+        {
+            // Per-app path: suppress core loopback, C# mixer feeds PCM.
+            var (result, streamId) = _client.StartStreamExternalFeed(VcStreamKind.ScreenAudio, "App audio");
+            if (result != VcResult.Ok)
+            {
+                AddActivity($"Failed to start screen audio: {result}");
+                return;
+            }
+            _screenStreamId = streamId;
+
+            _screenMixer = new ProcessAudioMixer();
+            _screenMixer.Start(scope, _client, streamId, picker.VisibleApps);
+
+            string desc = scope switch
+            {
+                OnlyApps    o => $"only {o.Pids.Count} app(s)",
+                AllExceptApps a => $"all except {a.Pids.Count} app(s)",
+                _ => "apps",
+            };
+            AddActivity($"Sharing screen audio: {desc}");
+        }
+
+        tsbScreenShare.Text = "Stop Screen Audio";
+        _miScreenShare.Text = "Stop Screen &Audio";
+    }
+
+    private void StopScreenAudio()
+    {
+        _screenMixer?.Stop();
+        _screenMixer?.Dispose();
+        _screenMixer = null;
+
+        _client.StopStream(_screenStreamId);
+        _screenStreamId = 0;
+        tsbScreenShare.Text = "Share Screen Audio";
+        _miScreenShare.Text = "Share Screen &Audio";
+        AddActivity("Stopped sharing screen audio");
+    }
+
     private void TrkOutputVolume_Scroll(object? sender, EventArgs e) =>
         _client.SetOutputVolume(trkOutputVolume.Value / 100f);
 
@@ -1019,7 +1066,7 @@ public partial class MainForm : Form
         _client.EventReceived -= OnEvent;
         foreach (var win in _pmWindows.Values.ToList()) win.Close();
         _pmWindows.Clear();
-        if (_screenStreamId != 0) _client.StopStream(_screenStreamId);
+        if (_screenStreamId != 0) { _screenMixer?.Stop(); _screenMixer?.Dispose(); _screenMixer = null; _client.StopStream(_screenStreamId); }
         if (_micStreamId != 0) _client.StopStream(_micStreamId);
         _client.Disconnect();
         _client.Dispose();

clients/windows/VoiceCat.App/VoiceCat.App.csproj

@@ -18,6 +18,9 @@
          WinForms' own analyzer (WFO0003) flags manifest-based DPI settings as superseded by
          this property in modern .NET. -->
     <ApplicationHighDpiMode>PerMonitorV2</ApplicationHighDpiMode>
+    <!-- WASAPI process-loopback COM interop in Audio/ProcessLoopbackCapture.cs uses unsafe
+         pointer arithmetic for PROPVARIANT/AUDIOCLIENT_ACTIVATION_PARAMS marshaling. -->
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   </PropertyGroup>
 
   <!-- System.Security.Cryptography.ProtectedData (PasswordProtector.cs's DPAPI wrapper) ships

clients/windows/VoiceCat.Interop/Structs.cs

@@ -46,6 +46,9 @@ internal struct VcStreamDescNative
     public IntPtr DeviceId; // unused by vc_stream_start today — device selection is a
                              // separate vc_set_input_device call; always IntPtr.Zero here.
     public IntPtr Label;
+    // Mirrors vc_stream_desc::external_feed. When 1, the core skips its own WASAPI loopback
+    // and the caller feeds PCM via StreamFeedPcm (per-app capture path on Windows).
+    public int ExternalFeed;
 }
 
 [StructLayout(LayoutKind.Sequential)]

clients/windows/VoiceCat.Interop/VoiceCatClient.cs

@@ -186,6 +186,26 @@ public sealed class VoiceCatClient : IDisposable
         }
     }
 
+    /// <summary>
+    /// Like <see cref="StartStream"/> but sets <c>external_feed = 1</c> so the core skips its
+    /// own WASAPI loopback. The caller is responsible for feeding PCM via
+    /// <see cref="StreamFeedPcm"/>. Used by the Windows per-app capture path.
+    /// </summary>
+    public (VcResult Result, uint StreamId) StartStreamExternalFeed(VcStreamKind kind, string label)
+    {
+        nint labelPtr = Marshal.StringToCoTaskMemUTF8(label);
+        try
+        {
+            var desc = new VcStreamDescNative { Kind = kind, DeviceId = 0, Label = labelPtr, ExternalFeed = 1 };
+            var r = NativeMethods.vc_stream_start(_handle.DangerousGetHandle(), in desc, out uint streamId);
+            return (r, streamId);
+        }
+        finally
+        {
+            Marshal.FreeCoTaskMem(labelPtr);
+        }
+    }
+
     public VcResult StopStream(uint streamId) =>
         NativeMethods.vc_stream_stop(_handle.DangerousGetHandle(), streamId);