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

From: martin rudalics
Subject: Re: jit-lock doesn't honor font-lock-lines-before
Date: Thu, 01 Sep 2005 08:24:14 +0200
User-agent: Mozilla Thunderbird 1.0 (Windows/20041206)

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

Then you mean that `font-lock-lines-before' is useless with jit-lock?

> ... 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.

When the "N previous lines" have not been fontified yet because they are
before `window-start' you do have to fontify them anyway.  Otherwise any
- syntactical, for example - context infomation needed to fontify the
current line correctly might be invalid.

>>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.


(defun foobar (bound)
  (while (re-search-forward "\\<foo\\>" bound 'bound)
      (when (re-search-forward "\\<bar\\>" (line-end-position 2) 'lep)
         (match-beginning 0) (match-end 0) 'face 'font-lock-warning-face)))))

(defun foobar-add ()
  (font-lock-add-keywords nil '(foobar . nil) t))

In some buffer do `foobar-add', write "foo" on one line and "bar" on the
next.  Plain font-lock will handle it, jit-lock won't.

>>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.

I don't know about font-lock-fontification-face-function.
`font-lock-multiline' is not a hack, at least not a priori.  It simply
has not been developed yet.  `font-lock-lines-before' is a brute hack.
However, until `font-lock-multiline' is implemented correctly, it might
be better than nothing.

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