procedure DoDrawChildrenDblBuffered( DC: HDC; WndParent: HWnd; const RectParent: TRect;
....
var R, CR: TRect;
    Save: Integer;
    P, P0: TPoint;
begin
  while W <> 0 do
  begin
    if IsWindowVisible( W ) then
    begin
....
      DoDrawChildrenDblBuffered( DC, W, CR, GetWindow( W, GW_CHILD ) );
      RestoreDC( DC, Save );
    end;
    W := GetWindow( W, GW_HWNDNEXT );
  end;
end;

function WndProcTransparent( Sender: PControl; var Msg: TMsg;
....
     case Msg.message of
         WM_ERASEBKGND: Result := TRUE;
         WM_PAINT:
         .....
                 Wnd := GetWindow( Sender.fHandle, GW_CHILD );
                 Wnd := GetWindow( Wnd, GW_HWNDLAST);
                 while Wnd <> 0 do begin
                     if IsWindowVisible(Wnd) then begin
                      ....
                     end;
                     Wnd := GetWindow( Wnd, GW_HWNDPREV );
                 end;
....