TreeView's SelectedItem in a Flyout gets set to null when the Flyout is closed

[FawazTakahji]

Describe the bug

If a top level item gets selected and the flyout gets closed the selecteditem gets set to null, this doesn't happen with a subitem

To Reproduce

1. Put a TreeView in a Flyout
2. Bind the SelectedItem to a property in the viewmodel
3. Select a top level item and close the flyout
4. SelectedItem will be set to null

Screenshots

Environment

• OS: I tested this on Windows 11 and WSL2 Ubuntu
• Avalonia-Version: 11.1.999-cibuild0043618-beta

Additional context

• I'm targeting .net 8.0
• example project

[timunie]

Maybe give #13970 a try?

[FawazTakahji]

Maybe give #13970 a try?

I gave it a try, by default it doesn't fix it, but if I set UpdateSourceTrigger to "LostFocus" it kinda fixes it

[goudcode]

Same when navigating to a different page.

https://github.com/AvaloniaUI/Avalonia/assets/23435053/13f8aebc-9061-4ec9-bb9b-69ff06117b8d

I have a sample project attached where this issue is reproducible.

treeviewtest.zip

[FawazTakahji]

As a temporary fix setting the binding mode for the ItemsSource to onetime fixes this issue on 11.0.7

People that use CommunityToolkit.Mvvm can do something like this

partial void OnSelectedItemChanged(Item? oldValue, Item newValue)
{
    if (newValue == null)
    {
        Dispatcher.UIThread.InvokeAsync(async () =>
        {
            await Task.Delay(3);
            SelectedItem = oldValue ?? Items[0];
        });
    }
}

if the property gets changed without a delay it keeps trying to set it to null until a stack overflow exception happens