SpamProtect.cs

public ObservableCollection<IInvoicePosition> Positions  // << dependency property
{ 
  get { /* ... */ }
  set { /* ... */ }
}

private volatile int _queue;
private readonly object _queueLock = new object();

private async void OnItemsSourceChanged(object sender, NotifyCollectionChangedEventArgs e)
{
    lock (_queueLock)
    {
        _queue++;
    }

    // I use a await instead of Thread.Sleep() to avoid blocking the UI thread
    // This might be the crux, any better way?
    await Task.Delay(100);

    lock (_queueLock)
    {
        _queue--;
    }
    if (_queue > 0) return;

    Draw();
}