I'm trying to set up multiline font-locking for a major mode I've been
working on. I've spent a fair bit of time wrestling with it, but I'm
still not getting completely desired results.
Emacs correctly fontifies the multiline region, and then correctly
refontifies it when the fontify-region function gets called again.

The problem is that when I make changes in the middle of the region,
the modified line and subsequent line get defontified until Emacs
comes through and refontifies.

When I set font-lock-multiline variable to 't', the text is never
defontified and new text is always immediately fontified correctly
(and without the aid of my region function [see below]). I'm guessing
then that there's some problem with my regexp or with the way I'm
applying my font-lock-multiline property.

What should I be doing differently so that I get the same behavior
without needing to turn font-lock-multiline on? Do I just _need_ to
set the font-lock-multiline variable to get the desired behavior?

Eric wrote:

Perhaps an alternative is to use multiple major modes:

http://www.emacswiki.org/cgi-bin/wiki/MultipleModes