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