MainWindow.xaml.cs

public partial class MainWindow : Window
{
    private DependencyObject _menuFocusScope;
    private DependencyObject _toolbarFocusScope;

    public MainWindow()
    {
        InitializeComponent();
    }

    private void Window_Loaded(object sender, RoutedEventArgs e)
    {
        _menuFocusScope = FocusManager.GetFocusScope(menuItemPaste);
        _toolbarFocusScope = FocusManager.GetFocusScope(buttonPaste);
    }  

    private void bindingPaste_PreviewCanExecute(object sender, CanExecuteRoutedEventArgs e)
    {
        IInputElement keyboardFocus = Keyboard.FocusedElement;

        if ((keyboardFocus != null) && (keyboardFocus != this))
        {
            DependencyObject pasteTargetAsDependencyObjeect = 
                keyboardFocus as DependencyObject;
            if (pasteTargetAsDependencyObjeect != null)
            {
                DependencyObject targetFocusScope = 
                    FocusManager.GetFocusScope(pasteTargetAsDependencyObjeect);

                if ((targetFocusScope != _menuFocusScope) && 
                    (targetFocusScope != _toolbarFocusScope) )
                {
                    menuItemPaste.CommandTarget = keyboardFocus;
                    buttonPaste.CommandTarget = keyboardFocus;
                }
            }
        }
    }
}