Overlay WPF Controls

I wrote a custom "auto-suggest" textbox control in WPF. It behaves a bit like the Hotmail "To" list, allowing the user to enter a list of items, offering suggestions whenever possible.

The main controls are a text box, a wrapper pane, and a list box. The text box captures user input. The wrapper pane contains a text box and shows previous entries. The list box is used to display suggestions. In most cases, the list box is hidden.

I am using multiple instances of my control in the stack pane.

My problem is when the list box is displayed, it is included in the control's height measurement. This causes the following controls in the stacking pane to move downward as these screenshots demonstrate:

alt text http://img13.imageshack.us/img13/8366/example1a.png

alt text http://img208.imageshack.us/img208/7664/example2a.png

I tried to override the dimension of my control to not include the list box, but that just results in the list box not showing.

I want to make the list box overlay any subsequent controls on the stack pane like a dropdown combo box. However, I really don't know how to do this. Any ideas?

thanks

Sandy