Starting with .net9.0 the ManagedThreadId for the UserInterface Thread is 2 and no longer 1

[Rand-Random]

Starting with .net9.0 the ManagedThreadId for the UserInterface Thread is 2 and no longer 1, I couldnt find documentation for this changed behaviour. Can you please point me to the change.

Simpe replication is:

UserInterface - e.g. MainWindow.xaml:

    <Grid>
        <TextBlock Name="ThreadId" />
    </Grid>

Code Behind:

    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
        protected override void OnContentRendered(EventArgs e)
        {
            base.OnContentRendered(e);

            ThreadId.Text = System.Threading.Thread.CurrentThread.ManagedThreadId.ToString();
        }
    }

Outputs:

Target Framework	ManagedThreadId (UI thread)
.NET Framework 4.8	1
.NET 8.0	1
.NET 9.0	2
.NET 10.0	2