bug#18793: 24.4; zero width rectangular selection displaces text

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Date: Wed, 22 Oct 2014 13:29:30 -0400
> Cc: 18793@debbugs.gnu.org
> 
> FWIW, I think the better option is to provide a way to draw a thin line
> that does not shift the rest of the text.

You could change the cursor to a vertical bar, and change its color to
match the color of the region.

Another possibility is to make a composition of the character at point
and some zero-width character (which we display as a thin space).  The
display engine already supports compositions, so the only problem is
to create one that will look "right", and solve the color issue in
some reasonable way.

Yet another possibility is to have a new cursor type that would
display a small portion of the block in another color.  This does
require changes to C code, but they are relatively simple and
straightforward.