PopupContent not updated when changing tabs in tabbed application

Consider a tabbed multi-document application, were the currently active document is ActiveDocViewModel.  I've got a ToolWindow like:

<actidock:ToolWindow Title="3D" x:Name="Panel3D" >
    <local:Panel3D DataContext="{Binding ElementName=dockSite, Path=DataContext.ActiveDocViewModel}" />
</actidock:ToolWindow>

In Panel3D, there's a listbox like:

<ListBox SelectionMode="Multiple" ItemsSource="{Binding SlicesList}" >
 <ListBox.ItemTemplate>
  <DataTemplate>
   <TextBlock Text="{Binding Name}"/>
  </DataTemplate>
 </ListBox.ItemTemplate>
 <ListBox.ItemContainerStyle>
  <Style TargetType="ListBoxItem">
   <Setter Property="IsSelected" Value="{Binding Path=IsChecked, Mode=TwoWay}"/>
  </Style>
 </ListBox.ItemContainerStyle>
</ListBox>

Everything above works as expected: the listbox shows the items in the SlicesList property, and tabs are switched, SlicesList is updated with the SlicesList property for the current document.  However, if I simply wrap the ListBox in a PopupButton:

<actishared:PopupButton Content="Select" DisplayMode="Merged">
 <actishared:PopupButton.PopupContent>
  <ListBox>
   <!--...Same as above...-->
  </ListBox>
 </actishared:PopupButton.PopupContent>
</actishared:PopupButton>

Suddenly, changing tabs no longer updates the SlicesList (all tabs show the same list of slices, belonging to the 1st tab).  Why would wrapping the ListBox in a PopupButton break the binding in this way?