But wait

The string protocol with xchars. New line has t men next line, otherwise if null meant end if line then the string needs a count.

The simplest is to accept '\n' as what it means, go to the next lines in the rectangle, (bump it by font heights plus margin).  So, no count in PutStr, it is terminated by a null and drops one line in the rectangle on a '\n'.

OK, getting this narrowed down,this last part, the default string to rectangle map. Once the string pointer is in p[lace, IO manager will call are draw string, as needed, he old string remaining, we presume.