NavigationItem: can't figure out how to set initial page

Describe the bug

I have a bunch of navigation items:

<wpfui:NavigationFluent
    x:Name="Navigation"
    Frame="{Binding ElementName=ContentFrame}"
    SelectedPageIndex="0">
    <wpfui:NavigationFluent.Items>
        <wpfui:NavigationItem
            Content="Title 1"
            Icon="Folder20"
            IconSize="20"
            PageType="{x:Type local:View1}" />
        <wpfui:NavigationItem
            Content="Title 2"
            Icon="Folder20"
            IconSize="20"
            PageType="{x:Type local:View2}" />
        <wpfui:NavigationItem
            Content="Title 2"
            Icon="Folder20"
            IconSize="20"
            PageType="{x:Type local:View3}" />
    </wpfui:NavigationFluent.Items>
</wpfui:NavigationFluent>

The navigation items work; if I click something in the sidebar, a page gets instantiated and shown, even with animation. However, I have SelectedPageIndex="0", so the first page should automatically show, and it does not. The pages do correctly get tags (PageTag is set), so that doesn't appear to be the issue.

I surmise from the code that there are two problems here:

1. UpdateServiceItems gets called too soon, so the NavigationService iterates over an empty collection.
2. It also doesn't initially have _frame set.

Both lead to it ultimately returning false.

I was able to hack around it with reflection as follows:

        var navigationBaseType = typeof(NavigationBase);
        _ = navigationBaseType.GetMethod("UpdateServiceItems", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance).Invoke(Navigation, null);
        var navigationService = navigationBaseType.GetField("_navigationService", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance).GetValue(Navigation);
        navigationService.GetType().GetField("_frame", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance).SetValue(navigationService, ContentFrame);

        _ = Navigation.Navigate("view1");

Now, View1 gets animated into place as soon as the window opens.

To Reproduce

Expected behavior

NavigationFluent should be sufficiently initialized such that either SelectedPageIndex or calling Navigate() in Loaded does something meaningful.

Screenshots

No response

OS version

Windows 11 23521

.NET version

.NET Framework 4.7.2

WPF-UI NuGet version

2.1.0

Additional context

No response

Trace NavigationFluent initialization through NavigationBase.UpdateServiceItems and NavigationService, focusing on when the item collection and _frame become available. Verify the behavior of SelectedPageIndex and Navigate() from Loaded using the reported XAML setup. Done means the initial page is shown without reflection, and navigation still works when items and the frame are initialized.