How is "VirtualizingObservableCollection.ReplaceAt" called?

[joshnoe]

I'm trying to have RaiseCollectionChangedEvent be called when my items are loaded into my datagrid. In the sample project, this happens, stemming from VizualizationManager.RunOnUI, which then calls VirtualizingObservableCollection.ReplaceAt.

However, in my project, VirtualizingObservableCollection.ReplaceAt isn't called when the data is bound. I've tried stepping through to see why, but there's so much indirection and the call stack is always lost due to Tasks etc, I've had no luck.

What causes VirtualizingObservableCollection.ReplaceAt to be called in the sample project?