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

[Stefan Monnier]

>> The use of `syntax-wholeline-max` in
>> `font-lock-extend-region-wholelines` supposedly fixed this problem since
>> it changed `font-lock` so it doesn't ask `syntax-ppss` to compute the
>> whole line/buffer.
>
> It did?
> And if it did, how is that better or different from a locked
> narrowing?

In terms of end-user behavior, it's very similar: it can break the
`font-lock-keywords` part of font-lock but it still lets `syntax-ppss`
look at the whole buffer and will thus still provide correct recognition
of strings and comments, except when the major mode relies on
`syntax-propertize-function` since that one also obeys
`syntax-wholeline-max` and can thus misbehave in a similar way to the
narrowing.

The more important difference is that it can be
tweaked/changed/broken/improved by any ELisp package without
necessitating a recompilation of Emacs's C code, or ugly workarounds to
escape the narrowing, like postponing the actual font-lock to a timer or
some such.


        Stefan