Thanks for the sample. There was one Trigger difference in the main DataGrid Style that we have here. In the real style, which more recently targets .NET 4.5, they have this Style trigger at the end:
<Condition Property="IsGrouping" Value="true" />
<Condition Property="VirtualizingPanel.IsVirtualizingWhenGrouping" Value="false" />
<Setter Property="ScrollViewer.CanContentScroll" Value="false"/>
That VirtualizingPanel.IsVirtualizingWhenGrouping property was added in .NET 4.5. Since our add-on still targets .NET 4.0, we can't reference it so you would need to clone the Style we provide in the open source there and adjust the Style trigger. Put the updated implicit Style in your app's Resources. That will get it working properly.