sp;          // Normal menu item.
           // Auto enable/disable if frame window has m_bAutoMenuEnable
           // set and command is _not_ a system command.
           state.m_pSubMenu = NULL;
           state.DoUpdate(this, FALSE);
        }

        // Adjust for menu deletions and additions.
        UINT nCount = pPopupMenu->GetMenuItemCount();
        if (nCount < state.m_nIndexMax)
        {
           state.m_nIndex -= (state.m_nIndexMax - nCount);
           while (state.m_nIndex < nCount &&
            pPopupMenu->GetMenuItemID(state.m_nIndex) == state.m_nID)
           {
            state.m_nIndex++;
           }
        }
        state.m_nIndexMax = nCount;
    }
}

更详细的信息可参考
http://support.microsoft.com/default.aspx?scid=kb;en-us;242577