Gerd Möllmann <gerd.moellmann@gmail.com> writes:

Nothing to add to your comments ATM, but maybe an alternative idea: how
about solving this with a new type of display property for drawing a
horizontal line? Say `horizontal-line WIDTH' where WIDTH could be nil or
something to draw a line to the end of the display area? That would have
the advantage that it would be a relatively locally confined change and
would have no impact when not used, and hopefully little potential to
break unrelated things when not used.

I like this idea.

One would have to have some Lisp though that puts the display property
on form-feeds.

In this case I would move this code to Lisp instead of C.