And finally with the lines truncated for real -- note the longer scroll
bar.

So it seems to compute the length of the scroll bar based on the
non-truncated length of the text in the buffer.

> What happens if you customize the value of
> scroll-bar-adjust-thumb-portion?

Then it seems to adjust the height of the scroll bar logically (i.e.,
based on what's shown in the window, and not on the length of the
(truncated) text).

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no