how would one do an IPad like overlay window like this
in WPF with XAML

I thought about a toggle button to or an expander control.
It would be nice if the panel is scaleable.
I have the most trouble with the centric overlay that has different size then the button itself.
Any help, links or resources would be great.
dowhilefor is correct, the Popup class is the way to go – I made a small sample project using a Popup with a custom control as the child. Of importance are the PlacementTarget and Placement fields of Popup, as they let you set where the popup appears. Hope this helps!
Custom control:
Window with popup:
Code behind for the window: