TabBar nests TabBarItems when being set as ItemTemplate's root

[kazo0]

With the following XAML

<utu:TabBar SelectedIndex="3"
            ItemsSource="{Binding Data.Items}"
            Style="{StaticResource TopTabBarStyle}">
    <utu:TabBar.ItemTemplate>
        <DataTemplate>
            <utu:TabBarItem Background="Red" Content="Content" />
        </DataTemplate>
    </utu:TabBar.ItemTemplate>
</utu:TabBar>

We should be seeing this:

Instead, we are seeing that that TabBar now contains TabBarItems that contain the red TabBarItem as it's content:

[Xiaoy312]

ChipGroup\Chip suffers the same problem

Both ChipGroup and TabBar inherits from ItemsControl, and both override GetContainerForItemOverride+IsItemItsOwnContainerOverride. This handles the case where the Items or ItemsSource contains the proper type for its item "container". However, it does not support the case where the supposed "container" is coming from the result of ItemTemplate.

That said, ListView and ListViewItem works... both on Windows and Uno somehow... On Uno, this is done via ItemsControl::GetRootOfItemTemplateAsContainer implemented in Selector. It is unclear how Windows is handling this..?