ffmpeg c# capture application with multiple monitors (screens) (no black screen)

ffmpegCaptureApplication.cs

private string AppName = "MyApp";

public void StartRecording()
{
    StopRecording();
    WinProcess.OnGetAllWindowsResult += WinProcess_OnGetAllWindowsResult;
    WinProcess.GetAllWindows(); //user32 EnumWindows
}

private void WinProcess_OnGetAllWindowsResult()
{
    WinProcess.OnGetAllWindowsResult -= WinProcess_OnGetAllWindowsResult;

    WinProcessRECT rect = new WinProcessRECT();
    var primaryScreenDPI = PresentationSource.FromVisual(this).CompositionTarget.TransformToDevice.M11;
    var primaryScreenWidth = (int)(SystemParameters.PrimaryScreenWidth * primaryScreenDPI);
    var primaryScreenHeight = (int)(SystemParameters.PrimaryScreenHeight * primaryScreenDPI);

    double appScreenDPI = primaryScreenDPI; //default
    Rect appScreenBounds = new Rect(0, 0, primaryScreenWidth, primaryScreenHeight); //default
    bool isAutogrammaFound = false;
    WinProcess.WinProc autogrammaWindow = default(WinProcess.WinProc);
    foreach (var wnd in WinProcess.GetAllWindowsResult)
    {
        if (wnd.Title == AppName)
        {
            autogrammaWindow = wnd;
            WinProcess.GetWindowExtenedeFrameBounds(wnd.Handle, ref rect); //Note: windows without invisible borders
            //=> dwmapi.dll DwmGetWindowAttribute(handle, (int)DwmWindowAttribute.DWMWA_EXTENDED_FRAME_BOUNDS, out lpRect, size);
            
            WinProcess.MonitorInfoEx appMonitorInfo = new WinProcess.MonitorInfoEx();
            
            var hMonitor = GetNearestMonitorToWindow(wnd.Handle); // => user32.dll MonitorFromWindow(hwnd, 0x00000002);
            WinProcess.GetMonitorInfo(hMonitor, appMonitorInfo); // => user32.dll GetMonitorInfo

            appScreenDPI = WinProcess.GetDisplayScaleFactor(wnd.Handle); // => user32.dll GetDpiForWindow(windowHandle) / 96f
            var dpiKoeff = appScreenDPI / primaryScreenDPI;
            appScreenBounds = new Rect(
                (int)(appMonitorInfo.WorkArea.Left * dpiKoeff), 
                (int)(appMonitorInfo.WorkArea.Top * dpiKoeff),
                (int)((appMonitorInfo.WorkArea.Right - appMonitorInfo.WorkArea.Left) * dpiKoeff),
                (int)((appMonitorInfo.WorkArea.Bottom - appMonitorInfo.WorkArea.Top) * dpiKoeff));


            if (rect.Left < -10000) //minimized
            {
                ;
            }
            else
            {
                isAutogrammaFound = true;
            }
            break;
        }
    }

    if (!isAutogrammaFound)
    {
        _errorMessage = "Окно "+AppName+" не найдено";
        StopRecording();
    }
    else
    {
        //Clamp
        rect.Right = (int)Clamp(rect.Right, appScreenBounds.Left, appScreenBounds.Right);
        rect.Left = (int)Clamp(rect.Left, appScreenBounds.Left, appScreenBounds.Right);
        rect.Bottom = (int)Clamp(rect.Bottom, appScreenBounds.Top, appScreenBounds.Bottom);
        rect.Top = (int)Clamp(rect.Top, appScreenBounds.Top, appScreenBounds.Bottom);

        WinProcess.SetForegroundWindow(autogrammaWindow.Handle);
        StartRecording(rect);
    }
}

private void StartRecording(WinProcessRECT rect)
{
    StopRecording();
    IsRecordingNow = true;

    var captureApp = "desktop";
    //var captureApp = "title=MyApp"; //note: black screen

    var width = rect.Right - rect.Left - 1;
    var height = rect.Bottom - rect.Top - 1;

    //need div 2 size (or error)
    if (width % 2 != 0)
        width -= 1;
    if (height % 2 != 0)
        height -= 1;

    _currentVideoPath = SaveFilePathWithDate;

    var args = $" -y -f gdigrab -offset_x {rect.Left} -offset_y {rect.Top} -video_size {width}x{height} -i {captureApp} {_vcodec} -vf \"setpts={(1f / _speed).ToString(CultureInfo.InvariantCulture)}*PTS\"  \"{_currentVideoPath}\"";

    try
    {
        var processStartString = _recordingExeFilePath + args;
        Debug.WriteLine(processStartString);
        _recordingProcess = WinProcess.Start(processStartString, Path.GetDirectoryName(_recordingExeFilePath), true); //DEBUG
    }
    catch (Exception ex)
    {
        Debug.WriteLine(ex);
        _errorMessage = "Ошибка записи";
        StopRecording();
    }
}