Re: jit-lock doesn't honor font-lock-lines-before

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

>>> The customizable variable `font-lock-lines-before' is not honored by
>>> jit-lock mode.
>> 
>> I know what that means in therms of what the code does, but I'm wondering in
>> which circumstance it makes a visible difference to the end user.
>> Do you have a test case?

> Suppose not: Wouldn't that mean `font-lock-lines-before' is useless?

Not at all.  font-lock-lines-before has not been introduced to force
refontification of the previous lines but because the N previous lines are
needed as context in order to properly refontify the current line.
Now the code does in fact refontify the previous lines, but it is
a side-effect rather than one of the original goals.

> It's not very difficult to contrive test cases for this.  By default
> `font-lock-multiline' is nil. `font-lock-fontify-anchored-keywords'
> won't alter it - the appropriate lines have been commented out.  Write
> an arbitrary multiline pattern.  Now font-lock won't give it the
> `font-lock-multiline' text property and jit-lock not necessarily reset
> its `fontified' text property after a change.  `font-lock-after-change',
> on the other hand, may refontify it provided `font-lock-lines-before' is
> large enough.

Please show me a test case.

> However, I believe that `font-lock-lines-before' is a brute force
> approach to handle such cases and could remarkably slow down editing if
> it were honored by jit-lock mode.  Multiline patterns are too delicate
> and should be treated in a completely different way.

Indeed, but nobody has put the work needed to handle them properly.
font-lock-multiline is one hack, font-lock-lines-before is another,
font-lock-fontification-face-function is yet another.


        Stefan