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

[Gregory Heytings]

> > I'm not sure I correctly understand what you mean, but it seems to me 
> > that the fact that the Org parser caches its results implies that Org 
> > doesn't need to access the whole buffer to make "local" decisions.  It 
> > can use the data in cache for everything that is outside of the current 
> > narrowing, and update the portion of the cache corresponding to the 
> > current narrowing. IOW, there is no need to maintain separate parser 
> > caches for each possible narrowing state.  Am I misunderstanding 
> > something?
>
> Cache existence is not guaranteed.

Can that cache not be created unconditionally when the Org file is opened?

> When cache does not exist, Org must widen. If not, the cache will be 
> valid only for the current narrowing.
>
> When cache do exist, Org may not need to widen. But that also mean that 
> widening has happened in the past when the cache was generated.
>
> At the end, Org does need to be able to widen safely. At least in some 
> cases.

That will always be possible.  What should better be avoided is to use 
widen (and to scan the whole buffer) in performance-critical places, such 
as fontification-functions.