Prototype of simple `SelectedContentControl` for housing content similarly to other controls like TabView/FlipView

SelectedContentControl.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;

namespace CommunityToolkit.WinUI.Controls.Future;

public sealed class SelectedContentControl : ListViewBase // Can't inherit from Selector: https://github.com/microsoft/microsoft-ui-xaml/issues/205
{
    public SelectedContentControl()
    {
        this.DefaultStyleKey = typeof(SelectedContentControl);

        UpdateSelection();

        SelectionChanged += SelectedContentControl_SelectionChanged;
    }

    protected override bool IsItemItsOwnContainerOverride(object item) => item is SelectedContentItem;

    protected override DependencyObject GetContainerForItemOverride() => new SelectedContentItem();

    protected override void PrepareContainerForItemOverride(DependencyObject element, object item)
    {
        base.PrepareContainerForItemOverride(element, item);

        if (element is SelectedContentItem container)
        {
            container.Visibility = (item == SelectedItem) ? Visibility.Visible : Visibility.Collapsed;
        }
    }

    private void SelectedContentControl_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        foreach (var removedItem in e.RemovedItems)
        {
            if (ContainerFromItem(removedItem) is SelectedContentItem removedContainer)
            {
                removedContainer.Visibility = Visibility.Collapsed;
            }
        }

        foreach (var addedItem in e.AddedItems)
        {
            if (ContainerFromItem(addedItem) is SelectedContentItem addedContainer)
            {
                addedContainer.Visibility = Visibility.Visible;
            }
        }
    }

    protected override void OnItemsChanged(object e)
    {
        base.OnItemsChanged(e);

        // Ensure a valid selection (auto-select first item)
        if ((SelectedIndex < 0 && Items.Count > 0)
            || SelectedIndex >= Items.Count)
        {
            SelectedIndex = 0;
        }

        UpdateSelection();
    }

    private void UpdateSelection()
    {
        // Show only the selected container; hide the rest.
        for (int i = 0; i < Items.Count; i++)
        {
            if (ContainerFromIndex(i) is SelectedContentItem container)
            {
                container.Visibility = (SelectedIndex == i) ? Visibility.Visible : Visibility.Collapsed;
            }
        }
    }
}

SelectedContentControl.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:controls="using:CommunityToolkit.WinUI.Controls.Future">

    <!--  Item container style  -->
    <Style x:Key="DefaultSelectedContentItemStyle" TargetType="controls:SelectedContentItem">
        <Setter Property="Padding" Value="0" />
        <Setter Property="HorizontalContentAlignment" Value="Stretch" />
        <Setter Property="VerticalContentAlignment" Value="Stretch" />
        <Setter Property="IsHoldingEnabled" Value="False" />
        <Setter Property="IsTabStop" Value="False" />
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="controls:SelectedContentItem">
                    <ContentPresenter HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                                      VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                                      Content="{TemplateBinding Content}"
                                      ContentTemplate="{TemplateBinding ContentTemplate}" />
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

    <Style BasedOn="{StaticResource DefaultSelectedContentItemStyle}" TargetType="controls:SelectedContentItem" />

    <!--  Control style  -->
    <Style x:Key="DefaultSelectedContentControlStyle" TargetType="controls:SelectedContentControl">
        <Setter Property="Background" Value="Transparent" />
        <Setter Property="BorderThickness" Value="0" />
        <Setter Property="CornerRadius" Value="0" />
        <Setter Property="IsSwipeEnabled" Value="False" />
        <Setter Property="IsTabStop" Value="False" />
        <Setter Property="SelectionMode" Value="Single" />
        <Setter Property="TabNavigation" Value="Once" />
        <Setter Property="UseSystemFocusVisuals" Value="{StaticResource UseSystemFocusVisuals}" />
        <Setter Property="ItemsPanel">
            <Setter.Value>
                <ItemsPanelTemplate>
                    <Grid />
                </ItemsPanelTemplate>
            </Setter.Value>
        </Setter>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="controls:SelectedContentControl">
                    <ScrollViewer x:Name="ScrollViewer"
                                  Background="{TemplateBinding Background}"
                                  BorderBrush="{TemplateBinding BorderBrush}"
                                  BorderThickness="{TemplateBinding BorderThickness}"
                                  CornerRadius="{TemplateBinding CornerRadius}"
                                  HorizontalScrollBarVisibility="Disabled"
                                  HorizontalScrollMode="Disabled"
                                  IsTabStop="False"
                                  VerticalScrollBarVisibility="Disabled"
                                  VerticalScrollMode="Disabled"
                                  ZoomMode="Disabled">
                        <ItemsPresenter />
                    </ScrollViewer>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

    <Style BasedOn="{StaticResource DefaultSelectedContentControlStyle}" TargetType="controls:SelectedContentControl" />
</ResourceDictionary>

SelectedContentItem.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

using Windows.UI.Xaml.Controls;

namespace CommunityToolkit.WinUI.Controls.Future;

public class SelectedContentItem : ListViewItem
{
    public SelectedContentItem()
    {
        DefaultStyleKey = typeof(SelectedContentItem);
    }
}

Discussion about the need for this and how this came about for XAML Studio in Windows App Community Discord here: https://www.discord.com/channels/372137812037730304/580484470877061120/1433743031764385925

Code was written/tested for UWP, but should work fine for WinUI 3 as well.