TWMMDIActivate = packed record
  Msg: Cardinal;
  case Integer of
    0: (ChildWnd: HWND;);
    1: (DeactiveWnd: HWND;
        ActiveWnd: HWND;
        Result: Longint;);
end;

struct TWMMDIActivate {
  unsigned Msg;
  HWND ChildWnd;
  HWND DeactiveWnd;
  HWND ActiveWnd;
  Longint Result;
};