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

[Richard M. Stallman]

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

That might be somewhat slower--but how much?  Would it even be
noticeable?

      jit-lock's after-change hook would only reset the
    fontified property to nil.  Whether and when these lines are refontified
    would be _also_ decided by the redisplay engine.

It's not decided by the redisplay engine alone.  When calls for
a certain region to be fontified, jit-lock could be changed to look
at the previous line too.