Handle requests from the editor<-scroll_bar.
Normally there is no reason to deal with this method yourself. We
document it here to explain the three scrolling models used by the
editor.
If the text_buffer<-size
(i.e. the number of characters held by the
editor) is large, It determines the position and size of the scrollbar
based on character offsets.
If the size gets medium, it counts physical lines, i.e.
the number of newline characters.
If the size gets small, it computes the line-layout, including
line-heigth using the current editor<-wrap
setting. This mode provides relyable precise scrolling, but quickly gets
too costly for real-time usage.
Inherits description from: editor<-length
