Wpf Textbox.AppendText is not showing in rum time

Dear Experts,

I'm using below code some process and i wish to show steps as finished task. the textbox is showing content run time but after finished the loop content is showing

What is missing

    Private Sub btnclose_Click(sender As Object, e As RoutedEventArgs) Handles btnclose.Click
        Dim x As Integer = 0
        For x = 0 To 18
            tbProgress.AppendText("Process Steps " + x.ToString + Environment.NewLine)
            System.Threading.Thread.Sleep(50)
        Next
    End Sub

Thanks & Regards Manoj Patel