WPF ListBox get current rendered index via ScrollViewer.ScrollChanged

<ListBox x:Name="lbx"
  ItemsSource="{Binding ImgsList,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
  SelectedIndex="{Binding SelectedIdx,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
  ScrollViewer.ScrollChanged="lbx_ScrollChanged">
    <behavior:Interaction.Triggers>
        <behavior:EventTrigger EventName="SelectionChanged">
            <behavior:InvokeCommandAction Command="{Binding SelectionChangedCommand}"
                                          CommandParameter="{Binding ElementName=lbx}"/>
        </behavior:EventTrigger>
    </behavior:Interaction.Triggers>          
    <ListBox.ItemTemplate>
        <DataTemplate>
            <Image Source="{Binding Path=DataContext,RelativeSource={RelativeSource 
                Mode=FindAncestor,AncestorType={x:Type ListBoxItem}}}"
                   Width="{Binding ActualWidth,RelativeSource={RelativeSource Mode=FindAncestor,
                AncestorType={x:Type Window}}}"
                   Height="{Binding ActualHeight,RelativeSource={RelativeSource Mode=FindAncestor,
                AncestorType={x:Type Window}}}"/>
        </DataTemplate>
    </ListBox.ItemTemplate>
</ListBox>

private ScrollViewer GetScrollViewer(DependencyObject dobj)
{
    if (dobj is ScrollViewer)
    {
        return dobj as ScrollViewer;
    }
    for (int i = 0; i < VisualTreeHelper.GetChildrenCount(dobj); i++)
    {
        var child = VisualTreeHelper.GetChild(dobj, i);
        var result = GetScrollViewer(child);
        if (result != null)
        {
            return result;
        }
    }
    return null;
}

private void lbx_ScrollChanged(object sender, ScrollChangedEventArgs e)
{
    ListBox listBox = sender as ListBox;
    if (listBox != null)
    {
        ScrollViewer scrollViewer = GetScrollViewer(listBox);
        if (scrollViewer != null)
        {
            int newIdx = (int)(scrollViewer.VerticalOffset / 3);

            if (newIdx != prevIdx)
            {
                Console.WriteLine($"prevIdx:{prevIdx} idx: {newIdx}");
                prevIdx = newIdx;
                Title = newIdx.ToString();
            }
        }
    }
}