display margins and modifying the buffer

[Juanma Barranquero]

From a two-month old discussion regarding display margins, overlays,
text properties and line numbers:

Me:

> but setting the display property of the overlay to show anything in the
> margin makes the text covered by the overlay go invisible;

Stefan Monnier:

> if you use overlays, it's not a big problem since you can make
> your overlay span 0 chars.

Finally I've had the time to try it and it does not work: overlays of
length 0 do not show anything in the margin. Making the span at least
length 1 does make the margin text appear, at the cost of making
invisible the text covered by the overlay.

I must be dense, but I don't see any way to use display margins to show
line-related information without modifying the buffer content or the way
its data is displayed.

                                                           /L/e/k/t/u