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

[Gregory Heytings]

> > 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).

And how much is "more"?  There's a reason it's called "jit-lock" and not 
"aot-lock", isn't it?

The docstring of jit-lock-mode is quite clear about this: it's a 
"demand-driven buffer fontification", "triggered by Emacs C code", with 
which "fontification occurs when necessary" when motion commands "would 
otherwise reveal unfontified areas".  Likewise "the START and END of the 
region to fontify" in the docstring of jit-lock-functions give the bounds 
within which fontification is supposed to happen.  Exceeding these bounds 
a bit, say by a few hundred characters, is okay; considering that they are 
mere hints and that the whole buffer can potentially be modified isn't.

> Furthermore, it's clear that fontification of BEG..END may need to look 
> at text before BEG (and occasionally beyond END as well).

Yes, and that's one the reasons why the locked narrowed region is in fact 
quite large.  For example, with emacs -Q, the size (width x height) of the 
window is 2880 characters, and the locked narrowed region is 16800 
characters, roughly equally distributed before and after point.  That's 
(inside a long line) about three screenfulls before and three screenfulls 
after point.