Keyboard navigation not working in WPF window inside Visual Studio

Hi,

I'm developing a VS-Package that shows a WPF window when a MenuCommand is invoked.

inside this WPF window I have a listBox, which is focused immediatly when the window is loaded.
when I show the window via  myWindow.ShowDialog(), the arrow up/down key work as expected as I navigate through the listbox items.

However, when I show the window via myWindow.Show(), the listbox gains focus but the keyboard navigation inside the listbox is NOT WORKING, and the PreviewKeyDown event is not firing.
(instead, the caret inside VS editor is moving).

Please help.

thank you!