Re: Major modes using `widen' is a good, even essential, programming practice.

[Stefan Monnier <monnier@iro.umontreal.ca>]

>> Eli Zaretskii [2022-08-07 17:20:52] wrote:
>> > jit-lock calls the functions with two arguments, BEG and END, and
>> > expects them to work only on that chunk of text.
>> 
>> That is not the case: it expects the function to "fontify" *at least*
>> from BEG to END, but is quite happy to let it fontify more (and the
>> function can return a value indicating which portion was actually
>> returned in that case).  Furthermore, it's clear that fontification of
>> BEG..END may need to look at text before BEG (and occasionally beyond
>> END as well).
>
> The intent is clearly that fontifications don't look far beyond these
> two points, because otherwise the whole design of jit-lock and its
> invocations during redisplay is basically thrown out the window.

Usually, font-lock rules don't look before BOL or after EOL, indeed,
*except* via `syntax-ppss` which does look at all the text from BOB
to point.  To make up for that, `syntax-ppss` relies heavily on caching,
so that it *usually* doesn't need to look very far at all (and if
there's no `syntax-propertize-function`, it's usually quite fast
because it's fully coded in C).

For GB-sized buffers, even the fast C code of `syntax-ppss` incurs
a significant delay in the "unusual" case, so have various options:
- suck it up (potentially wait several minutes when jumping to the end
  of the file).
- give up providing more or less correct highlighting (either via some
  arbitrary narrowing like we do now, or turning off font-lock).
- try and find some clever heuristic that can find a "nearby safe spot",
  i.e. a position for which we can guess the PPSS value (usually we
  look for a position that is "known" to be outside of any string,
  comment, or parenthesis).
- display the buffer quickly without highlighting while the fontification
  is computed in the background.


        Stefan