Even in the self-test suite, the bug appears, run:

python /usr/lib/python2.4/curses/textpad.py

then type so that the line wraps, eg 123456789123456789 to fill two 
lines, and then ctrl-g to return -- the result has had the end 
characters of each line removed, so the return value is 
12345678\n12345678\n

As a quick hack on my local install I changed line 56 from:

last = min(self.maxx, last+1)

to

last = min(self.maxx, last)+1

and it seems to work, but I have no idea if this is the right way to 
fix it (ie, this might have side effects, or this off-by-one might 
happen in several places and need a global fix, not just one local one)