Can anyone think of a way in Emacs, today, to visually indicate the start
and end of an overlay or a stretch of a face text property, as suggested by
the little red corners in the first attached image? Notice the part
highlighted in yellow, which shows two adjacent blue highlights. (This is
from http://graphics.csail.mit.edu/~rcm/chi04.pdf, which Herbert/Guanpeng
cited in his email today: "A prototype of intelligent replace for Emacs".)

IOW, how might one, today, indicate two adjacent overlays (or text-property
zones), so that a user can see the boundaries of each? Is this possible? If
not, how might Emacs be made to support this (after the release)?

Such "corners" would be useful. If it were not feasible to implement such a
feature, could we perhaps implement something similar using box faces with
borders? That is, could we have a way to make a bordered-box face show the
vertical border at each overlay beginning and end? See the second attached
image for what I mean.

Today, if I use a box face with a border, and I create an overlay from L to
M and another overlay from M+1 to N, then the appearance is the same as a
single overlay from L to N. It would be useful to be able to specify that
the two overlays each show their vertical borders at beginning and end. This
wouldn't be as clear as having corners, as in the first screenshot, but it
would still be useful.

WDOT?