In my mind, there is no question that 784 is the right answer. That being
said (and in such an absolute fashion), I can understand how other people
would see things differently.
I guess a more succinct way to ask your question is "does back mean go back
where you came from, or does it mean re-traverse the last link you
traversed before you got here".
If one really wanted to be pedantic about this, one could say that the real
meaning of scrolling down is to pretend you are traversing a link to the
next line in the document. Thus, in your case, doing the back would get
you to line 784, and doing additional backs would scroll you in reverse
back to 699, and then finally jump you back to 58. I doubt, however, that
that's the behavior people would expect (or want) to see.