Returns the range of lines which are currently visible to the user. Lines are numbered starting with 0.
The result is only valid when the buffer is viewed (see Basic Organization). Calling this method may force the recalculation of location information, even if the buffer currently has delayed refreshing (see refresh-delayed?).
The start box is filled with first line visible to the user, unless start is null. The end box is filled with last line visible to the user, unless end is null.