WindowFinder.xaml.cs

            var mouseDowns = Observable.FromEventPattern<MouseButtonEventHandler, MouseButtonEventArgs>(h => MouseDown += h, h => MouseDown -= h);
            var mouseUps = Observable.FromEventPattern<MouseButtonEventHandler, MouseButtonEventArgs>(h => MouseUp += h, h => MouseUp -= h);

            mouseDowns.Merge(mouseUps).WhenAny() .Subscribe(OneMouseUp);
        }

        private void OneMouseUp(EventPattern<MouseButtonEventArgs> eventPattern)
        {
            var windowUnderCursor = NativeMethods.GetWindowUnderMouse();

            Messenger.Default.Send(new WindowInfo(windowUnderCursor));
        }

Forget the WhenAny()