The WM_NCMOUSELEAVE message is posted to a window when the cursor leaves the nonclient area of the window specified in a prior call to TrackMouseEvent.
A window receives this message through its WindowProc function.
WM_NCMOUSELEAVE
WPARAM wParam
LPARAM lParam;
Parameters
wParam
Not used, must be zero.
lParam
Not used, must be zero.
Return Value
If an application processes this message, it should return zero.
Remarks
All tracking requested by TrackMouseEvent is canceled when this message is generated. The application must call TrackMouseEvent when the mouse reenters its window if it requires further tracking of mouse hover behavior.
MSDN里说的很明显了。WH_GETMESSAGE Hook只拦截由GetMessage or PostMessage的消息，不能拦截由SendMessage引起的消息。
he WH_GETMESSAGE hook enables an application to monitor messages about to be returned by the GetMessage or PeekMessage function. You can use the WH_GETMESSAGE hook to monitor mouse and keyboard input and other messages posted to the message queue.
要Hook SendMessage必须Hook WH_CALLWNDPROC and WH_CALLWNDPROCRET Hooks
The WH_CALLWNDPROC and WH_CALLWNDPROCRET hooks enable you to monitor messages sent to window procedures by the SendMessage function. Windows calls a WH_CALLWNDPROC hook procedure before passing the message to the receiving window procedure, and calls the WH_CALLWNDPROCRET hook procedure after the window procedure has processed the message.
The WH_CALLWNDPROCRET hook passes the address of a CWPRETSTRUCT structure to the hook procedure. The structure contains the return value from the window procedure that processed the message, as well as the message parameters associated with the message. Subclassing the window does not work for messages set between processes.
