Why does this code deadlock?

Why does this code deadlock?

I created a simple WPF project with a single button, and this is the button click code.

If I comment out the SetSynchronizationContext then it works. (Which brings up another question, why doesn't it crash complaining I'm not on a STA thread for the second MessageBox, because after the await it resumes on a different thread.)

With the SetSynchronizationContext line, it seems to deadlock at the await statement, I'm guessing it's unable to resume on the same thread it started on. Is there a hidden .Join() or .Wait() somewhere blocking the thread it started on? Is the thread it started on blocked somehow? (Or is it gone?)

private void Button_Click(object sender, RoutedEventArgs e)
{
    // Create a thread
    Thread newWindowThread = new Thread(new ThreadStart(async () =>
    {
        MessageBox.Show("Initial ThreadId=" + Thread.CurrentThread.ManagedThreadId.ToString());
        SynchronizationContext.SetSynchronizationContext(new DispatcherSynchronizationContext());
        await DoAsync();
        MessageBox.Show("Now ThreadId2=" + Thread.CurrentThread.ManagedThreadId.ToString());
    }));
    newWindowThread.SetApartmentState(ApartmentState.STA);
    newWindowThread.IsBackground = true;
    newWindowThread.Start();
}

private async Task DoAsync()
{
    await Task.Delay(1000);
}

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