How to bind index of current page of ItemsRepeater in WinUI 3?

I have ScrollViewer that contain ItemsRepeater of images and I want to bind in TextBox the current index page of ItemsRepeater when scroll the images. this is my .xaml code:

<StackPanel Orientation="Horizontal">
    <TextBlock>
        <Run Text="{x:Bind ViewModel.Pages.Count, Mode=OneWay}"/>
        <Run Text=" / "/>
    </TextBlock>
    <TextBox Text="{x:Bind ViewModel.PageIndex, Mode=TwoWay}" LostFocus="PageIndexTextBox_LostFocus" x:Name="PageIndexTextBox" InputScope="Number"/>
</StackPanel>
<ScrollViewer x:Name="scroll" ZoomMode="Enabled" Grid.Row="1"
              IsTabStop="True" IsVerticalScrollChainingEnabled="True"
              HorizontalAlignment="Stretch" VerticalAlignment="Top"
              HorizontalScrollMode="Enabled" HorizontalScrollBarVisibility="Hidden"
              VerticalScrollMode="Enabled" VerticalScrollBarVisibility="Auto"
              ViewChanged="ScrollViewer_ViewChanged">
    <ItemsRepeater ItemsSource="{x:Bind ViewModel.Pages, Mode=OneWay}" 
                   MaxWidth="{Binding ElementName=scroll, Path=ActualWidth}"
                   HorizontalAlignment="Stretch"
                   VerticalAlignment="Stretch"
                   x:Name="PdfImageRepeater">
        <ItemsRepeater.ItemTemplate>
            <DataTemplate x:DataType="pdfImage:PdfImage">
                <Grid Padding="5" HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
                    <Image Source="{x:Bind Image, Mode=OneWay}" Stretch="Uniform" 
                           HorizontalAlignment="Center" VerticalAlignment="Center"/>
                </Grid>
            </DataTemplate>
        </ItemsRepeater.ItemTemplate>
        <ItemsRepeater.Layout>
            <StackLayout Orientation="Vertical"/>
        </ItemsRepeater.Layout>
    </ItemsRepeater>
</ScrollViewer>

And this is in my .xaml.cs code:

   private void PageIndexTextBox_LostFocus(object sender, RoutedEventArgs e)
   {
       if (int.TryParse(PageIndexTextBox.Text, out int pageIndex))
       {
           pageIndex -= 1;

           if (pageIndex >= 0 && pageIndex < ViewModel.Pages.Count)
           {
               var pageItem = PdfImageRepeater.GetOrCreateElement(pageIndex);
               pageItem.UpdateLayout();
               pageItem.StartBringIntoView();
           }
       }
   }

How can I add option to display index of current ScrollViewr page in WinUI 3 ?

If you can fix the height of the items, for example:

<ItemsRepeater.ItemTemplate>
    <DataTemplate x:DataType="local:Member">
        <Grid
            MinHeight="50"
            MaxHeight="50">
            <!-- Item Content  -->
        </Grid>
    </DataTemplate x:DataType="local:Member">
</ItemsRepeater.ItemTemplate>

you can do something like this:

private ScrollBar? VerticalScrollBar { get; set; }

private void ItemsRepeaterScrollViewer_Loaded(object sender, RoutedEventArgs e)
{
    if (sender is not ScrollViewer scrollViewer ||
        scrollViewer.FindDescendant<ScrollBar>(x => x.Orientation is Orientation.Vertical) is not ScrollBar verticalScrollBar)
    {
        return;
    }

    VerticalScrollBar = verticalScrollBar;
    VerticalScrollBar.ValueChanged += VerticalScrollBar_ValueChanged;
}

private void VerticalScrollBar_ValueChanged(object sender, RangeBaseValueChangedEventArgs e)
{
    MemberIndexNumberBox.Value = Math.Round(e.NewValue / 50);
}

private void MemberIndexNumberBox_ValueChanged(NumberBox sender, NumberBoxValueChangedEventArgs args)
{
    if (VerticalScrollBar is null)
    {
        return;
    }

    ItemsRepeaterScrollViewer.ScrollToVerticalOffset(args.NewValue * 50);
}

The FindDescendant extension comes from the CommunityToolkit.WinUI.Extensions NuGet package.