AutocompleteBox - DropDownHeight when Window is small?

I have an AutoCompletebox on a Window that is very narrow.  The Window Height=60.

This means that the PopUp/ListBox drops down outside the window. 

As you can see in the image if I make the Height of the PopUp more then it increases in height.  This is the black you see, but the list box does not. 

I have tried my same style on a higher test window and I get my desired dropdown height so I have narrowed my issue down to the window height being too small.

So my question, what is restricting the Listbox to a certain size when the hosting window is not very high?