Does simulating click affect GetMessagePos()?

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I want to select one item of CTreeCtrl by sending message
WM_LBUTTONDOWN to it, but the CTreeCtrl's message processing
function uses API GetMessagePos() to fetch the last message's
position which is always not the one corresponding to my simulating
WM_LBUTTONDOWN. The CTreeCtrl's message processing function
further uses CTreeCtrl::HitTest() to check whether that item was clicked,
and inevitably HitTest() returns NULL.

My code is as follows:

   POINT ap={0x0027, 0x0289}, cp={0x0025, 0x00AE};
   ScreenToClient((HWND)hTreeView, &ap);
   cp = ap;
   SendMessage((HWND)hTreeView, WM_LBUTTONDOWN, MK_LBUTTON, cp.y<<16|cp.x);

The screen coordinates (0x0027, 0x0289) are absolutly correct, because
I have used SetCursorPos() to check them. And I have also tried the
following code to activate the tree item, yet it was selected but not
activated(its corresponding code was not executed):

   SendMessage((HWND)hTreeView, TVM_SELECTITEM, TVGN_CARET, hLstRetract);

So the problem lies in GetMessagePos(). I guess this API ignores any
simulating message, it only returns the position of the last physical
message say one actual click. Does anyone have an idea to defeat
GetMessagePos()? Thank you.