Re: C-n is very slow in Font-Lock mode

["Eli Zaretskii" <eliz@gnu.org>]

> Cc: emacs-devel@gnu.org
> From: David Kastrup <dak@gnu.org>
> Date: Wed, 27 Apr 2005 00:57:05 +0200
> 
> Without looking at the code, it would be my guess that it would
> currently run immediately after redisplay, whereas my suggestion for
> jit-lock-defer-time=0 semantics would be that it be run immediately
> before redisplay.

I'm not sure what that means.  You _are_ aware that, when you scroll
thru text with C-n or C-v, there's almost nothing going on in Emacs
_except_ redisplay, yes?  That is, all C-n does is move point one
line; the rest is redisplay's job, including fontification.  (The
single most important new aspect of JIT lock as opposed to the old
lazy-lock and its ilk was that JIT lock uses special hooks of the
display engine, whereas the old, pre-v21 redisplay didn't have such
hooks, and so lazy-lock needed to jump through the hoops to hook into
every possible method of changing the displayed text.)

So when you are saying ``before redisplay'', what exactly do you mean?
Redisplay begins by computing the ``desired glyph matrix'', which is
an internal representation of what should be on the screen; surely,
before this happens, there's no way we could fontify the right portion
of the text, since we have no idea what that portion is.

> > And fontification of the visible portion will take time during which
> > one still sees unfontified text, anyway.
> 
> Not if it happens before redisplay. [...]
> 
> It would possibly make sense to implement this distinction in the
> context of run-with-idle-timer, and then let jit-lock-defer-time just
> use it by defaulting to -1.

Idle timers run when Emacs is idle, which by definition is _after_
redisplay did its job.  So I think you are talking about some nw
mechanism, which has nothing to do with how jit-lock-defer-time works
now.

> Again: I have not looked at the code.

Well, perhaps you should.