GetComboBoxInfo button rectangle size

Hi

I've got a problem with the GetComboBoxInfo function in C++ when Windows XP
or Vista themes are on. The rectangle returned in the COMBOBOXINFO.rcButton
is too small by 1 pixel at each edge. This is causing a problem in one of my
overriding controls, as the dropdown has been customised. The problem is that
when the user clicks exactly on the edge of the button part of the combobox,
the default windows dropdown is showing because my check to see whether or
not the button was clicked is returning false.

Note that there are no problems if XP themes are off. Is there a workaround
for this problem? If not, I need to check whether or not the modern themes
are on (but I'm not sure how to do that at the moment). Can someone help me
out?

Here's a code snippet

bool CMatrixCtrl::IsMouseOnButton(const CPoint &screenpoint) const
{
  CRect rCombo;
  GetWindowRect(rCombo);
  if (rCombo.PtInRect(screenpoint))
  {
    // combobox info
    COMBOBOXINFO cbinfo;
    cbinfo.cbSize = sizeof(COMBOBOXINFO);
    GetComboBoxInfo(&cbinfo);
 
    CPoint point = screenpoint;
    ScreenToClient(&point);
    // check if point is inside the combo button
    CRect rButton(cbinfo.rcButton);
    if (rButton.PtInRect(point)) // This is returning false when XP themes
are on and the edge of the button is clicked.
    {
      return true;
    }
  }
  return false;
}

I am using Windows Vista and MFC 8 with Visual Studio 2005.