How to fix direction of combo box pop up in silverlight?

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C#

XAML

Silverlight

I have a combobox in a pod which expands and collapses. When the pod is in collapsed position, combo box comes at the bottom of the screen and when i click on it the popup pops sometimes above the combo box(which is required) but sometimes it pops up in downward direction, hiding it from the view. Is there any way i can fix the direction of the pop up so that it opens in the upward direction when the pod is in collapsed state.

What I have tried:

Went through blogs but could not find a solution