Parchment ZVM: centering a line of text

Authoring Inform 6
1

With an older version of Parchment and ZVM, I could center a line of text with the font off and spaces instructions but it seems that this is no longer possible with the recent version.

Code 
Constant HDR_SCREENWCHARS  $21;
Array Storage buffer 160;

[ ScreenWidth;
    return (HDR_SCREENWCHARS->0);
];

[ StrLen str;
   @output_stream 3 Storage;
   print (string) str;
   @output_stream -3;
   return Storage-->0;
];

[ CenterLine str   len width cx;
   len = StrLen(str);
   width = ScreenWidth();
   cx = (width - len) / 2;
   font off;
   spaces cx;
   print (string) str;
   new_line;
   font on;
];

[ main key;
   CenterLine("* CENTERED TEXT *");
   @read_char 1 ->key;
];