You aren’t reading a byte at a time, it’s a block. You have no guarantee that you just ended on yet another partial sequence. It usually makes more sense to just rewind to the last complete sequence (though with streams that is not always an option).
> You have no guarantee that you just ended on yet another partial sequence
Which is why you don't read to the end (until you have an EOF). You can just sidestep the partial sequence problem by getting more data before there's potential for a partial sequence to show up. The longest possible sequence is 4 bytes, so as long as you ensure you have 4 bytes available, partial sequences are impossible on well formed input.
