Re: something like linum.el ought to be added

[Richard Stallman]

    I see what you mean; the main complication is that existing overlays
    can still be necessary in other windows that show the same buffer.

That is true.

                                                                       So
    making all overlays available for (re-)consumption would entail
    walking all visible windows and adjusting (new or existing) overlays
    where necessary,

Indeed, the after-change-functions hook will need to deal with
all the windows.  That is not very hard, with buffer-window-list.

If each overlay records which windows it is meant for, the
window-scroll-functions can could delete and redo only those that
pertain to the current window.  Each window will run the
window-scroll-functions to recompute the overlays it needs.

(We can add a primitive to select overlays based on a certain
property value, to make that faster.)

We could also add this as a display feature.
It might not be very hard.