Re: something like linum.el ought to be added

[Richard Stallman]

    > Preserving overlays on the other parts of the buffer won't things
    > speed up

    Since those overlays can be reused when re-visiting the parts of the
    buffer they were created for, this can speed things up since it avoids
    re-creation of overlays and intermediate garbage collection.

Someone pointed out that you can use the same set of overlays
regardless of the buffer position.

    Line numbers should also be updated after scrolling, M-x goto-line and
    many other functions that change the visible portion.

That is true.  But you can do that by putting a hook onto
window-scroll-functions.  That plus after-change-functions let you get
a call for every event that could make it necessary.

Take a look at jit-lock.el and see how it determines when
to act and where.  linum.el can do it the same way.