Re: something like linum.el ought to be added

[Richard Stallman]

    > I think that means we need to add a suitable hook which redisplay can
    > call to inform Lisp code that visibility changes may have altered the
    > position of the bottom of the window.

    OTOH, using jit-lock would make this unnecessary.

jit-lock implements a specific functionality, fontification, and
nothing else.

I thought that the hooks used by jit-lock ought to be sufficient
for linum as well.  But it turns out they are not, because
the code to run Vfontification_functions checks specifically
for the `fontified' property.

We could add another similar hook, or a more general mechanism to
implement similar hooks for various uses.  But I think that is the
wrong way to go, because it depends on putting text properties on the
text, to indicate which parts have been processed.  (Fontified, in the
case of jit-lock.)

We don't want linum to put on text properties to say where it has
put the line numbers.  So I think my idea of using the same hooks
jit-lock uses was a mistake.

We seem to have another solution, which I suggested and Markus Triska
implemened.