专门用于消息回调窗口的窗口标识HWND_MESSAGE（创建一个非可视、没有z-order的窗口）

HWND_MESSAGE 
Message-Only Windows

A message-only window enables you to send and receive messages. It is 
not visible, has no z-order, cannot be enumerated, and does not 
receive broadcast messages. The window simply dispatches messages. To 
create a message-only window, specify the HWND_MESSAGE constant or a 
handle to an existing message-only window in the hWndParent parameter 
of the CreateWindowEx function. You can also change an existing window 
to a message-only window by specifying HWND_MESSAGE in the 
hWndNewParent parameter of the SetParent function. To find 
message-only windows, specify HWND_MESSAGE in the hwndParent parameter 
of the FindWindowEx function. In addition, FindWindowEx searches 
message-only windows as well as top-level windows if both the 
hwndParent and hwndChildAfter parameters are NULL. 
see: https://msdn.microsoft.com/en-us/library/windows/desktop/ms632599(v=vs.85).aspx

示例: 
HWND hWnd = CWindowImpl::Create(HWND_MESSAGE, CWindow::rcDefault, NULL, WS_POPUP, 0, (UINT)0);

To create a message-only window, supply HWND_MESSAGE or a handle to an existing message-only window.

https://msdn.microsoft.com/en-us/library/windows/desktop/ms632599(v=vs.85).aspx#message_only