bug#56682: Fix the long lines font locking related slowdowns

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: 56682@debbugs.gnu.org,  Eli Zaretskii <eliz@gnu.org>,  Dmitry Gutov
>  <dgutov@yandex.ru>
> Date: Wed, 03 Aug 2022 18:42:03 -0400
> 
> I don't care about the mishighlighting and find the current behavior
> perfectly acceptable from an end-user point of view.  I only care about
> the extra enforcement done in C code without providing any mechanism to
> circumvent it.  Especially since this discussion seems to suggest that
> if I were to propose a patch that makes this locking a bit more "soft",
> it might be rejected on the grounds that it opens the door to abuse

"Show me the code", instead of imagining what would be the reaction to
that.  (To say nothing of the fact that what I already wrote several
times explicitly contradicts your impression.)