Mishandling \b at the edge of the window

A user reported an issue:

Open standard 80 column window and do this

$ printf "%080d\b\n" 0
00000000000000000000000000000000000000000000000000000000000000000000000000000000
$ printf "%080d\b \n" 0
000000000000000000000000000000000000000000000000000000000000000000000000000000 0

Notice that 79th zero is erased instead of 80th, which is wrong.

Edited Oct 10, 2018 by Igor

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