the wpf datagrid scrollbar is

Subscription to LayoutUpdated in Loaded handler solves the problem. In this case IsLoaded check is also redundant. So, having

public FancyDataGrid() : base()
{
    Loaded += OnLoaded;
}

private void OnLoaded(object sender, RoutedEventArgs e)
{
    LayoutUpdated += OnLayoutUpdated;
}

instead of

public FancyDataGrid() : base()
{
    LayoutUpdated += OnLayoutUpdated;
}

allows to access viewport when it’s calculated with regard to vertical scrollbar.