Repro case for bxc#52604 (adjust #if in Exit to test both cases)

AppDelegate.cs

using System;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using System.Threading;

using AppKit;
using Foundation;

namespace UnableToExit
{
    [Register (nameof (AppDelegate))]
    sealed class AppDelegate : NSApplicationDelegate
    {
        [DllImport ("libc")]
        static extern void exit (int c);

        [DllImport ("libc")]
        static extern void atexit (IntPtr a);

        static Action atExitDelegate;

        public override void DidFinishLaunching (NSNotification notification)
        {
            Log ();

            atExitDelegate = AtExit;
            atexit (Marshal.GetFunctionPointerForDelegate (atExitDelegate));

            new Thread (BackgroundThread).Start ();
        }

        void BackgroundThread (object state)
        {
            Log ();
            InvokeOnMainThread (Exit);
        }

        void Exit ()
        {
#if false
            Log ("invoking Environment.Exit");
            Environment.Exit (0);
#else
            Log ("invoking exit");
            exit (0);
#endif
        }

        void AtExit () => Log ();

        void Log (string message = null, [CallerMemberName] string caller = null)
            => Console.WriteLine (
                "thread {0}: {1} {2}",
                Thread.CurrentThread.ManagedThreadId,
                caller,
                message);
    }
}

Results.md

Output when exiting via exit:

thread 1: DidFinishLaunching 
thread 4: BackgroundThread 
thread 1: Exit invoking exit
thread 1: AtExit

Expected output when exiting via Environment.Exit:

thread 1: DidFinishLaunching 
thread 4: BackgroundThread 
thread 1: Exit invoking Environment.Exit
thread 1: AtExit

Actual output (does not exit, hangs in Environment.Exit):

thread 1: DidFinishLaunching 
thread 4: BackgroundThread 
thread 1: Exit invoking Environment.Exit