Hi, I'm calling it like this just to test the behavior:
<Popup x:Name="abc" IsOpen="True">
<Popup.ChildTransitions>
<TransitionCollection>
<PaneThemeTransition />
</TransitionCollection>
</Popup.ChildTransitions>
<Grid Background="AliceBlue" Width="700" Height="100" />
</Popup>
This is placed in the main page grid. I toggle the IsOpen property with a button. When I toggle it to on it animates, but when I toggle it off it just disappears.
I have tried to view the Popup sample, but the page says the item is not yet published.