ItemsRepeater scrolls to wrong element

[grokys]

Describe the bug

ItemsRepeater scrolls to the wrong element when scrolled to end and trying to bring into view last item.

Steps to reproduce the bug

Edit BasicDemo to contain the following:

    <ScrollViewer HorizontalScrollBarVisibility="Disabled"
                      VerticalScrollBarVisibility="Auto"
                      Grid.Row="1" Width="250" Height="350">
        <Border Background="LightBlue">
            <controls:ItemsRepeater x:Name="repeater">
                <controls:ItemsRepeater.ItemTemplate>
                    <DataTemplate>
                        <Border BorderBrush="Red" BorderThickness="1">
                            <TextBlock HorizontalAlignment="Stretch" Text="{Binding}"/>
                        </Border>
                    </DataTemplate>
                </controls:ItemsRepeater.ItemTemplate>
            </controls:ItemsRepeater>
        </Border>
    </ScrollViewer>

    public sealed partial class BasicDemo : Page
    {
        public BasicDemo()
        {
            this.InitializeComponent();
            repeater.ItemsSource = Enumerable.Range(0, 10000).Select(x => x.ToString()).ToList();
        }

        protected override void OnPointerPressed(PointerRoutedEventArgs e)
        {
            base.OnPointerPressed(e);

            var element = repeater.GetOrCreateElement(9999);
            element.UpdateLayout();
            element.StartBringIntoView();
        }
    }

Run the program and manually scroll to the end (using scrollbar). Click an item to trigger OnPointerPressed. ItemsRepeater scrolls to item 19 despite requesting item 9999.

Expected behavior

Nothing happens because item 9999 is already visible.

Screenshots

Version Info

Current master (a53a650)

Windows 10 version	Saw the problem?
Insider Build (xxxxx)
May 2019 Update (18362)	Yes
October 2018 Update (17763)
April 2018 Update (17134)
Fall Creators Update (16299)
Creators Update (15063)

Device form factor	Saw the problem?
Desktop	Yes
Mobile
Xbox
Surface Hub
IoT

[jevansaks]

Sounds very similar to #1423, maybe same root cause?

[grokys]

Quite possibly, I was unsure whether to put them both in the same issue, but I figured it would be easier to track in two issues if they turn out to be different bugs.

[grokys]

The problem seems to go something like this:

• repeater.GetOrCreateElement(9999); is called
• ItemsRepeater::GetOrCreateElementImpl() calls m_viewportManager->OnMakeAnchor() with the element, meaning m_makeAnchorElement is set
• When the control is measured, at first anchorRealized == true
• Then m_elementManager.OnBeginMeasure() is called
• This gets the realization rect from m_context.RealizationRect()
• Which ends up in ViewportManagerWithPlatformFeatures::GetLayoutVisibleWindow()
• Which sees that m_makeAnchorElement is set and so returns a visible window starting at 0, 0
• OnBeginMeasure() passes this rect to DiscardElementsOutsideWindow
• Which discards the anchor element as it thinks it's outside the window!

So it seems like this comment, while maybe true for non-laid out anchors, does the precise opposite for laid-out anchors:

microsoft-ui-xaml/dev/Repeater/ViewportManagerWithPlatformFeatures.cpp

Lines 102 to 105 in e7e0823

	// The anchor is not necessarily laid out yet. Its position should default
	// to zero and the layout origin is expected to change once layout is done.
	// Until then, we need a window that's going to protect the anchor from
	// getting recycled.

[marcelwgn]

I would like to look into this issue.