What’s wrong with this code, part 25

Wow, 25 already.

This one’s pretty straightforward.  Once again, it’s a UI issue, since I’ve been spending most of my time doing UI lately.

In this particular case, the code comes from the constructor for an auto-layout class that is used internally in one of our tools.  It saves away window handles for a control and the dialog which holds the control, then saves the size of the dialog and relative location of the control within that dialog.  There’s other code that handles resizing and adjusting the layout of the control when the dialog is resized. 

 CControlLayout::CControlLayout(const HWND hWndControl, const HWND hWndDlg)
    : m_hWnd(hWndControl)
    , m_hWndDlg(hWndDlg)
{
    // Get the parent (dialog) rect, and the control rect
    ::GetClientRect(m_hWndDlg, &m_rcRefDlg);
    ::GetWindowRect(m_hWnd, &m_rcRef);
    ScreenToClientRect(hWndDlg, m_rcRef);
}

 void ScreenToClientRect(/* [in] */ const HWND hWndClient, 
                        /* [in/out] */ RECT &rcInOut)
{
 CPoint ptTopLeft(rcInOut.left, rcInOut.top);
 CPoint ptBottomRight(rcInOut.right, rcInOut.bottom);

  ::ScreenToClient(hWndClient, &ptTopLeft);
 ::ScreenToClient(hWndClient, &ptBottomRight);

  rcInOut.left = ptTopLeft.x;
 rcInOut.top = ptTopLeft.y;
 rcInOut.right = ptBottomRight.x;
 rcInOut.bottom = ptBottomRight.y;
}

m_rcRefDlg holds the reference rect for the dialog and m_rcRef holds the reference rect for the control relative to the dialog.

This code has been in the UI for quite a while and recently one of our testers discovered a nasty bug while running a test pass.

The question is: What’s wrong with this code.  I believe we shipped Windows Vista with this bug (I’m not 100% sure, since I don’t have a Windows Vista machine to test it), so it’s pretty subtle.

Edit: OOPS - I forgot to include ScreenToClientRect.