CWnd* SetSysModalWindow();
Makes the CWnd a system-modal window.
If another window is made the active window (for example, the system-modal window creates a dialog box that becomes the active window), the active window becomes the system-modal window. When the original window becomes active again, it is system modal. To end the system-modal state, destroy the system-modal window.
A pointer to the window object that was previously the system-modal window. The returned pointer may be temporary.
::SetSysModalWindow, CWnd::GetSysModalWindow