bug#66732: tree-sitter fontification doesn't update multi-line syntax reliably

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Dmitry Gutov <dmitry@gutov.dev>,  66732@debbugs.gnu.org,
>   dominik@honnef.co,  casouri@gmail.com
> Date: Mon, 18 Dec 2023 22:40:59 -0500
> 
> >> It seems like in our scenario the region does not get re-rendered even 
> >> when some of the faces changed in there.
> > Can't happen.
> 
> I think Dmitry is talking about the case where the faces were change
> after the text was rendered (but before the end of the redisplay cycle).
> 
> I.e. the redisplay starts, renders up to position POS, then calls
> jit-lock because `fonfitied` is nil, and jit-lock modifies faces of text
> *before* POS.

This can only happen if a function called from jit-lock doesn't comply
to the protocol wrt such changes, right?  Because otherwise the
modified region would have been re-rendered on the next redisplay
cycle at the latest.