How Do I Overlay And Increase Height Of Panel In Wpf

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WPF

Need some direction with the creation of a control in WPF, the interface is like below:
The control would come before the image control.
The control contents are not fully displayed, if the user wants to view full details, user can click on the button on the control to make it expand.However it should overlay on the image below and not push it down.

| _________Control__________________|
|
|
|___________________________________|


| _______________Image______________|
|
|
|
| __________________________________| <- Image section


I have tried to expander control:
1. It shows or hides all the content.
2. Push down the image below.

Probably i would need to create custom control,which derives from ContentControl,
how would i add the expanding tray like capability with a higher zindex.

Any help is appreciated.