Fixes an issue that can cause Avalonia-based applications to crash. T…

…he GetMessage call in RunLoop() of WinUiCompositorConnection.cs has been updated to process the result of GetMessage, and terminate the message loop if a WM_QUIT message is received on the pump. (#17190)

Presently the WM_QUIT message is ignored and the loop re-enters the GetMessage call, causing an unmanaged exception and process crash.

#17188

src/Windows/Avalonia.Win32/WinRT/Composition/WinUiCompositorConnection.cs

@@ -161,12 +161,20 @@ private void RunLoop()
             return UnmanagedMethods.DefWindowProc(hwnd, msg, w, l);
         });
         UnmanagedMethods.SetTimer(dw.Handle, IntPtr.Zero, 1000, null);
-        while (!cts.IsCancellationRequested)
+
+        var result = 0;
+        while (!cts.IsCancellationRequested
+               && (result = UnmanagedMethods.GetMessage(out var msg, IntPtr.Zero, 0, 0)) > 0)
         {
-            UnmanagedMethods.GetMessage(out var msg, IntPtr.Zero, 0, 0);
             lock (_shared.SyncRoot)
                 UnmanagedMethods.DispatchMessage(ref msg);
         }
+
+        if (result < 0)
+        {
+            Logger.TryGet(LogEventLevel.Error, "WinUIComposition")
+                ?.Log(this, "Unmanaged error in {0}. Error Code: {1}", nameof(RunLoop), Marshal.GetLastWin32Error());
+        }
     }
 
     public static bool IsSupported()