If I understand you correctly, you have 3 "windows" in your application, 1 of which is a container. The container should then open 1 of the other windows in the stackpanel. If this is the case, the best way to do this is to make the other 2 windows UserControls instead. These can then be hosted in your stackpanel, 1 with its Visibilty set to Visible and 1 set to Collapsed - this means you'll only see 1 item in the stackpanel at a time. Then, use a trigger to change the Visibility of the items based on the menu click.
Deja View - the feeling that you've seen this post before.
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