Use loopcancellation instead of adding a seperate api.

native/Avalonia.Native/inc/avalonia-native.h

@@ -308,7 +308,6 @@ AVNCOM(IAvnPlatformThreadingInterface, 0b) : IUnknown
     // Can't pass int* to sharpgentools for some reason
     virtual void Signal(int priority) = 0;
     virtual IUnknown* StartTimer(int priority, int ms, IAvnActionCallback* callback) = 0;
-    virtual HRESULT TerminateApp () = 0;
 };
 
 AVNCOM(IAvnSystemDialogEvents, 0c) : IUnknown

native/Avalonia.Native/src/OSX/platformthreading.mm

@@ -138,12 +138,6 @@ public:
             return new TimerWrapper(callback, ms);
         }
     }
-    
-    virtual HRESULT TerminateApp () override
-    {
-        [NSApp stop:0];
-        return S_OK;
-    }
 };
 
 @implementation Signaler

src/Avalonia.Native/PlatformThreadingInterface.cs

@@ -45,6 +45,7 @@ namespace Avalonia.Native
 
         readonly IAvnPlatformThreadingInterface _native;
         private ExceptionDispatchInfo _exceptionDispatchInfo;
+        private IAvnLoopCancellation _loopCancellation;
 
         public PlatformThreadingInterface(IAvnPlatformThreadingInterface native)
         {
@@ -59,36 +60,31 @@ namespace Avalonia.Native
 
         public void RunLoop(CancellationToken cancellationToken)
         {
-            if (cancellationToken.CanBeCanceled == false)
-                _native.RunLoop(null);
-            else
+            var l = new object();
+            _loopCancellation = _native.CreateLoopCancellation();
+            cancellationToken.Register(() =>
             {
-                var l = new object();
-                var cancellation = _native.CreateLoopCancellation();
-                cancellationToken.Register(() =>
+                lock (l)
                 {
-                    lock (l)
-                    {
-                        cancellation?.Cancel();
-                    }
-                });
-                try
-                {
-                    _native.RunLoop(cancellation);
+                    _loopCancellation?.Cancel();
                 }
-                finally
+            });
+            try
+            {
+                _native.RunLoop(_loopCancellation);
+            }
+            finally
+            {
+                lock (l)
                 {
-                    lock(l)
-                    {
-                        cancellation?.Dispose();
-                        cancellation = null;
-                    }
+                    _loopCancellation?.Dispose();
+                    _loopCancellation = null;
                 }
+            }
 
-                if(_exceptionDispatchInfo != null)
-                {
-                    _exceptionDispatchInfo.Throw();
-                }
+            if (_exceptionDispatchInfo != null)
+            {
+                _exceptionDispatchInfo.Throw();
             }
         }
 
@@ -99,7 +95,7 @@ namespace Avalonia.Native
 
         public void TerminateNativeApp()
         {
-            _native.TerminateApp();
+            _loopCancellation?.Cancel();
         }
 
         public void Signal(DispatcherPriority priority)