Why does my screengrabber grab the transient highlight from the menu item that invoked it?

void test::OnEditCopyImage()     // Copy to the Clipboard.
{
    bool clipboardAcceptedData = true;
    
    // Capture the window to a bitmap.
    COXScreenGrabber ScreenGrabber; 
    RECT rect;
      
    mainFrame->SetWindowPos(&CWnd::wndTopMost,0,0,0,0,SWP_NOMOVE|SWP_NOSIZE|SWP_SHOWWINDOW);
    GetParent()->GetWindowRect(&rect);  // get the rect of the frame, not just the graph
    
    ScreenGrabber.GrabRectangle(rect);
    COXDIB* pDib = ScreenGrabber.GetGrabDIB();
   
    VERIFY(OpenClipboard());
    VERIFY(EmptyClipboard()):// Remove the current Clipboard contents & reassign ownership.
    if (::SetClipboardData(CF_DIB, pDib->m_hDIB)==NULL)
    {
        AfxMessageBox(IDS_STRING123);
        clipboardAcceptedData = false;
    }
    
    CloseClipboard();
    //keep the bitmap 'alive', the clipboard now owns it.
    // Set the handle to NULL so that the destructor does not free the memory.
    if ( clipboardAcceptedData == true )
        pDib->m_hDIB = NULL; 
}