Using the PopupButton from within a WPFToolkit DataGrid immediately closes the popup

Hi,

I am using the PopupButton from within a DataGrid column.  When the popup is opened, it immediately closes again.  Interestingly, this only happens if you click the PopupButton using the left mouse button - using the right mouse button works fine.

Does anyone have any ideas?

<Controls:DataGrid x:Name="TestDataGrid"
                    AutoGenerateColumns="False"
                    CellEditEnding="TestDataGrid_CellEditEnding">
    <Controls:DataGrid.Columns>
        <Controls:DataGridTemplateColumn Header="Test Column">
            <Controls:DataGridTemplateColumn.CellTemplate>
                <DataTemplate>
                    <Shared:PopupButton Content="Test" >
                        <Shared:PopupButton.PopupMenu>
                            <ContextMenu>
                                <MenuItem Header="Option 1" />
                                <MenuItem Header="Option 2" />
                            </ContextMenu>
                        </Shared:PopupButton.PopupMenu>
                    </Shared:PopupButton>
                </DataTemplate>
            </Controls:DataGridTemplateColumn.CellTemplate>
        </Controls:DataGridTemplateColumn>
    </Controls:DataGrid.Columns>
</Controls:DataGrid>