Program hangs when I use trackPopupMenuEx

## Current Behavior
Program hangs with trackPopupMenu(Ex).

## Steps to Reproduce (for bugs)

My code is based on https://github.com/haskell/win32/blob/v2.6.1.0/examples/hello.lhs, except I do not do painting.

``` haskell
wndProc ::
Graphics.Win32.HWND
-> Graphics.Win32.WindowMessage
-> Graphics.Win32.WPARAM
-> Graphics.Win32.LPARAM
-> IO Graphics.Win32.LRESULT
wndProc hwnd wmsg wParam lParam
| wmsg == Graphics.Win32.wM_COMMAND && wParam == fromIntegral iDM_QUIT = do
Graphics.Win32.destroyWindow hwnd
return 0
| wmsg == 0x007B = do
showContextMenu hwnd
return 0
| wmsg == Graphics.Win32.wM_DESTROY = do
Graphics.Win32.sendMessage hwnd Graphics.Win32.wM_QUIT 1 0
return 0
| otherwise = do
Graphics.Win32.defWindowProc (Just hwnd) wmsg wParam lParam

showContextMenu :: Graphics.Win32.HWND -> IO ()
showContextMenu hwnd = do
hMenu <- createPopupMenu
pt <- Graphics.Win32.Misc.getCursorPos
appendMenu hMenu mFT_STRING iDM_Exit (Just "&Exit")
trackPopupMenuEx hMenu tPM_RIGHTBUTTON (fromIntegral $ fst pt) (fromIntegral $ snd pt) hwnd Nothing
destroyMenu hMenu
```
A pop-up menu appears, but the program ends up freezing after that. I've tried to remove `WM_COMMAND` branch from `wndProc`, but this does not help.

## Your Environment

* I'm using stack, tried resolvers lts-6.2, lts-16.8
* Versions used: 2.3.1.1, 2.6.1.0
* Operating System and version: Windows 10 Home