The advantages of being able to scroll so that the last line can be moved
to the top can be questioned though. It is not something other applications
do. Usually when the the last line is visible the thumb is at the bottom
and you can not scroll so that the last line is moved to the top.
But there is explicit code in Emacs for this behaviour so I figured
that it had been decided some time ago. It should perhaps be a
settable customization?
Actually, this is also difficult to do. What is the window-start
position that ensures that point-max is visible ? The only
safe choice is when window-start == point-max.