FlowLayout: Rapid wheel scrolling variable-sized ItemsRepeater causes layout-cycle crash

Describe the bug

Microsoft.UI.Xaml.Controls.FlowLayout enters a layout cycle when an ItemsRepeater contains variable-sized items and is scrolled quickly with the mouse wheel. The layout iteration countdown reaches zero and the app terminates with 0xC000027B.

This reproduces with the current experimental WinUI 3 implementation in Microsoft.WindowsAppSDK 2.4.1-experimental.

Why is this important?

FlowLayout cannot safely virtualize a basic collection whose items have different widths and heights. A normal rapid wheel-scroll gesture can terminate the application, so applications cannot use this layout for variable-sized content.

Steps to reproduce the bug

1. Create a packaged C# WinUI 3 app referencing Microsoft.WindowsAppSDK 2.4.1-experimental.
2. Replace MainWindow.xaml and MainWindow.xaml.cs with the minimal code below.
3. Run the app.
4. Move the pointer over the ScrollViewer.
5. Scroll quickly with the mouse wheel.

<Window
    x:Class="FlowLayoutCycleRepro.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:FlowLayoutCycleRepro">
    <ScrollViewer
        Width="520"
        Height="320"
        HorizontalScrollMode="Disabled"
        VerticalScrollMode="Enabled">
        <ItemsRepeater ItemsSource="{x:Bind Items}">
            <ItemsRepeater.Layout>
                <FlowLayout />
            </ItemsRepeater.Layout>
            <ItemsRepeater.ItemTemplate>
                <DataTemplate x:DataType="local:ReproItem">
                    <Border
                        Width="{x:Bind Width}"
                        Height="{x:Bind Height}"
                        Background="SteelBlue">
                        <TextBlock
                            HorizontalAlignment="Center"
                            VerticalAlignment="Center"
                            Text="{x:Bind Label}" />
                    </Border>
                </DataTemplate>
            </ItemsRepeater.ItemTemplate>
        </ItemsRepeater>
    </ScrollViewer>
</Window>

using Microsoft.UI.Xaml;
using System.Collections.Generic;
using System.Linq;

namespace FlowLayoutCycleRepro;

public sealed partial class MainWindow : Window
{
    public IReadOnlyList<ReproItem> Items { get; } = Enumerable.Range(1, 500)
        .Select(index => new ReproItem(
            $"Item {index}",
            72 + index % 4 * 20,
            52 + index % 3 * 12))
        .ToList();

    public MainWindow()
    {
        InitializeComponent();
    }
}

public sealed record ReproItem(string Label, double Width, double Height);

No navigation, event handlers, custom title bar, or application-specific layout code is involved.

Actual behavior

Rapid wheel scrolling produces:

Layout Iteration Countdown: 7. Launching Arrange Pass.
Layout Iteration Countdown: 6. Launching EffectiveViewport Pass.
Layout Iteration Countdown: 5. Raising LayoutUpdated Events.
Layout Iteration Countdown: 4. Launching Measure Pass.
Layout Iteration Countdown: 3. Launching Arrange Pass.
Layout Iteration Countdown: 2. Launching EffectiveViewport Pass.
Layout Iteration Countdown: 1. Raising LayoutUpdated Events.
Layout Iteration Countdown: 0. Launching Measure Pass.
Crash: Exception (0xC000027B)

Expected behavior

The collection scrolls and FlowLayout continues realizing and arranging items without entering a layout cycle.

NuGet package version

Microsoft.WindowsAppSDK 2.4.1-experimental

Windows version

Windows build 26200 (ARM64)

Additional context

• The repro uses the default FlowLayout; no optional layout properties are needed.
• The failure reproduces with 500 items whose widths and heights vary.
• A 100-item variant did not reproduce on the first tested wheel gesture.
• microsoft/microsoft-ui-xaml#2607 reported the same general symptom on the older WinUI stack and was closed as stale. This issue provides a current, minimal, deterministic WinUI 3 repro.

Start by running the provided MainWindow.xaml and MainWindow.xaml.cs reproduction with Microsoft.WindowsAppSDK 2.4.1-experimental, then inspect the FlowLayout and ItemsRepeater layout paths involved in scrolling and variable-sized items. Done means rapid wheel scrolling no longer reaches the layout iteration limit or terminates the app, while the collection continues realizing and arranging items.