Opening a new Window with InputGestures: main window steals focus

I have a simple UserControl which executes a routed command when user double clicks:

<UserControl.InputBindings>
    <MouseBinding Command="someRoutedCommand" Gesture="LeftDoubleClick" />
</UserControl.InputBindings>

Somewhere upper in the visual tree this routed command is bound to the procedure:

<UserControl.CommandBindings>
    <CommandBinding Command="someRoutedCommand" Executed="someProc"/>
</UserControl.CommandBindings>

The procedure opens a new window:

private void someProc(object sender, ExecutedRoutedEventArgs e)
{
    var win = new SomeWindow();
    win.Show();
    //win.Activate(); 
}

The problem is that the new window doesn't remain active. It become active for a while and then the main window acivates again. How can I avoid this?

I've just realised tahat this problem occures only if i embed the control in a TreeView. If it is places just in a Window's Grid it works OK.

ansaurus