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

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Eli Zaretskii <eliz@gnu.org>,  56682@debbugs.gnu.org
> Date: Mon, 01 Aug 2022 03:11:43 -0400
> 
> >> Your current code makes it impossible for a major mode to make Emacs slow
> >> by widening in a too-long-line.
> > Which is a good thing, isn't it?
> 
> No: we don't want to prevent people from shooting themselves in the foot.

But the problem here is that it isn't "people shooting themselves in
the foot", it's that "major modes shoot their users in the foot".
IOW, the ones who shoot and the ones who get shot are not the same
people.

What do you want a user to do when he/she is faced with a mode which
makes Emacs very slow?  Such a user cannot blame his/herself; in many
cases the use doesn't even know enough to realize it's the major mode
and its fontifications that are the culprit.

> Widening in a too long line will not necessarily lead to an unresponsive
> Emacs, so by preventing it you're also preventing useful cases.

The experience up till now says otherwise.