Causes reentrant by invalid monitor lock usage with async/await method.

MainWindow.xaml

<Window x:Class="WpfApp2.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <DockPanel>
        <Button DockPanel.Dock="Bottom" Click="Button_Click" Content="Append" />
        <TextBlock x:Name="input" />
    </DockPanel>
</Window>

MainWindow.xaml.cs

using System.Diagnostics;
using System.IO;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Windows;

namespace WpfApp2
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private async void Button_Click(object sender, RoutedEventArgs e)
        {
            // DANGEROUS DIRTY HACK: It's going to ignore multiple locking at single main thread. (It's nonsense and cause reentrants)
            // lock (this)
            Monitor.Enter(this);
            try
            {
                Trace.WriteLine($"Begin: Thread={Thread.CurrentThread.ManagedThreadId}");

                using (var fs = new FileStream("output.txt", FileMode.Append, FileAccess.Write, FileShare.None, 65536, true))
                {
                    var tw = new StreamWriter(fs, Encoding.UTF8);

                    // Maybe causes reentrant below awaiters by owned main thread.
                    await tw.WriteLineAsync("Clicked.");
                    await Task.Delay(1000);   // Simulate long awaited code.
                    await tw.FlushAsync();
                }

                Trace.WriteLine($"End: Thread={Thread.CurrentThread.ManagedThreadId}");
            }
            finally
            {
                Monitor.Exit(this);
            }
        }
    }
}