Full ItemSource iteration on ExtendSelection

https://github.com/dotnet/wpf/blame/4ae9e616402ba1128eb11fb075746ed9b2fd6269/src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/DataGrid.cs#L4845

FullRow Extending Selection traverses all `ItemSources` from index 0 to `endIndex`, stacking the UI if the `ItemSource` contains a large number of items.

For example, there are more than 1,000,000 rows and the selection starts somewhere at the end of the datagrid. Colossal load on the ItemsSource. In my case, `ItemsSource` is a virtualised collection, so if an item was not in the collection, a request is sent to the database to load it.

So I end up having to find a workaround, set `DataGrid.SelectionMode == Single`, and simulate `Extended` mode in my code.

Please, say, for what enumerator is here, and may it be fixed?