Return the number of lines that scrolled to above the top of terminal {buf}. This is the offset between the row number used for term_getline() and getline(), so that:

term_getline(buf, N)

is equal to:

getline(N + term_getscrolled(buf))

(if that line exists).

{buf} is used as with term_getsize().

Can also be used as a method:

GetBufnr()->term_getscrolled()