https://learn.microsoft.com/en-us/dotnet/api/system.collections.specialized.notifycollectionchangedeventargs.newstartingindex?view=netframework-4.8.1&viewFallbackFrom=net-9.0
NotifyCollectionChangedEventArgs.NewStartingIndex Property (System.Collections.Specialized) |...
Gets the index at which the change occurred.
property systemcollectionsspecialized