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

[Dmitry Gutov <dgutov@yandex.ru>]

On 14.08.2022 19:24, Gregory Heytings wrote:
> 
>>
>> So being able to fine-tune the behavior makes sense. Certainly not a 
>> "maintenance nightmare".
>>
> 
> If the conclusion is, after some reasonable effort, that there is no way 
> to make syntax-ppss significantly faster in one way or another in such 
> cases, and that there is no way to make font locking reasonably accurate 
> even when it doesn't have access to the whole buffer, it might make 
> sense to provide user options to fine-tune the behavior.  But we are not 
> there yet.

Both conclusions lead to removing the applications of narrowing from 
handle_fontified_prop. So how about we either do that (defaulting to 
accurate font-lock), or merge the branch I proposed, and then continue 
on to the more complex developments?

Implementing the "font locking reasonably accurate even when it doesn't 
have access to the whole buffer" would also have to be implemented in 
Lisp, so narrowing outside of font-lock doesn't make sense.