The part of the scroll bar that's supposed to behave like C-v
or <PageDown> is not working right: sometimes it doesn't scroll
as far as C-v would scroll, and sometimes it scrolls further.

A scroll command that goes too far can be dangerous: it can put
you in a situation where you think you've seen all of the lines
in a document, but in reality you haven't, because some of them
were never visible on your screen while you were paging through
the document.

Attached are screen shots taken before and after a click that
incorrectly scrolled by more than a complete windowful, causing
3½ lines to be hidden from the user.