It is a new window coming up, and half the window (the top of it) is given a small amount of transparency to.
You can try to add a new StackPanel with Black background whose Alpha value is set to .4 to your current window, and fill the bottom 80% with a child that would have a Solid background with the content on it, the remaining top content would have a Transparent theme because the StackPanel would have the transparency.
The content can be aligned at the bottom using
<stackpanel verticalalignment="Bottom" background="White">
You would be showing or hiding the
in your application.