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

[martin rudalics]

>     Because this means that every time I insert one character, redisplay
>     would refontify `font-lock-lines-before' in addition to the current
>     line.
>
> Would that be bad?

It would slow down editing long lines.  jit-lock is supposed to make
editing faster.

>
>       Whether and when these lines are refontified
>     would be _also_ decided by the redisplay engine.  And the latter doesn't
>     know about `font-lock-lines-before'.  For example, editing the first
>     line in a window would not "include the line before in refontification".
>
> This is a good point.  However, it can probably be fixed by a change in
> the fontification function--when it is called, it should extend the starting
> point for refontification upward by the specified number of lines.
> (This might be the only fix needed.)
>
>

In `font-lock-default-fontify-region', immediately before the comment

          ;; check to see if we should expand the beg/end area for
          ;; proper multiline matches

insert either

          (setq beg (line-beginning-position (1- font-lock-lines-before)))

or - but this would have to be mentioned in the doc-string of
`font-lock-lines-before' -

          (when font-lock-multiline
            (setq beg (line-beginning-position (1- font-lock-lines-before))))

remove the corresponding line from `font-lock-after-change-function' and
wait what happens?  I'm still sceptical, though ... fontifications would
always overlap by `font-lock-lines-before' lines.