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

[Stefan Monnier]

>> > Indeed, that's my conclusion too.  So until syntax-ppss (at least) is made
>> > an order of magnitude faster, the right thing to do is to use the forced
>> > narrowing method.
>> 
>> By tying this to "long lines" is wrong, since it has nothing to do with
>> long lines, only with large buffers.
>
> I thought you told me once that syntax-propertize needs to consider
> complete lines in some (frequent) situations?

Yes, but we're talking about `syntax-ppss` here.  Admittedly,
`syntax-ppss` uses `syntax-propertize` internally, but I think the two
need to be considered separately (and `syntax-propertize` already tries
to bound its work via `syntax-wholeline-max`).


        Stefan