Full ItemSource iteration on ExtendSelection

[lolomarka]

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?

[singhashish-wpf]

@lolomarka Could you please share a sample repro and proper repro steps? We would also like to see the workaround which you employed along with the problem details. It would be great if you could use the issue template and fill out the details.