`FindName()` does not work when root object is a Window

[HO-COOH]

Describe the bug

FindName() does not work in WinUI3, while the exact same code works in UWP.

Steps to reproduce the bug

1. Create a new C++WinUI3 packaged project
2. Use this xaml

   <StackPanel x:Name="Root" Orientation="Vertical" HorizontalAlignment="Center" VerticalAlignment="Center" Spacing="5">
       <Button x:Name="myButton" Click="myButton_Click">Click to load</Button>
       <Rectangle x:Name="DelayLoadRect" Fill="Green" Width="100" Height="50" x:Load="False" Loaded="Rectangle_Loaded"/>
   </StackPanel>

3. Use this code

   void MainWindow::myButton_Click(IInspectable const&, RoutedEventArgs const&)
   {
       static bool IsLoaded = false;
       if (IsLoaded)
       {
           winrt::Microsoft::UI::Xaml::Markup::XamlMarkupHelper::UnloadObject(DelayLoadRect());
           myButton().Content(box_value(L"Click to load"));
       }
       else
       {
           myButton().Content(box_value(L"Click to unload"));
           auto delayLoadRect = Root().FindName(L"DelayLoadRect");
       }
       IsLoaded = !IsLoaded;
   }
   
   void MainWindow::Rectangle_Loaded(winrt::Windows::Foundation::IInspectable const& sender, winrt::Microsoft::UI::Xaml::RoutedEventArgs const& e)
   {
       winrt::Microsoft::UI::Xaml::Controls::ContentDialog dialog;
       dialog.XamlRoot(Content().XamlRoot());
       dialog.Title(winrt::box_value(L"Loaded"));
       dialog.PrimaryButtonText(L"OK");
       dialog.ShowAsync();
   }

4. Build and run, the green rect is not loaded. The dialog is not shown.

Repro here

Expected behavior

No response

Screenshots

No response

NuGet package version

WinUI 3 - Windows App SDK 1.5.5: 1.5.240627000

Windows version

Windows 11 (22H2): Build 22621

Additional context

No response

[eduardobragaxz]

Try it on a page instead.

[HO-COOH]

Try it on a page instead.

Moving it to a Page does work, but I think the fact that it does not work under Window needs to be documented.