[racket-dev] pr 12683 and using something like text:nbsp->space?

I'm trying to prepare a patch to fix pr 12683 (zero-width space).
Currently, I've got:

    https://github.com/dyoo/racket/commit/ed6570d482acada2027988903c11e08ffac03e3c

However, at this point, I get stuck, because I have no idea where to
mix-in this into the DrRacket text% instance.  I did greps for
'nsbp->space' throughout the collects tree to see where it must be
normalizing, but grep is failing me as well.  Where should I be
looking?  Thanks!