bug#12937: 24.2; Fontification performed in 'jit-lock-defer-time' in any case (don't wait while Emacs become really idle).

[Oleksandr Gavenko <gavenkoa@gmail.com>]

I open (a very long .c file):

  emacs-bzr/trunk/src/coding.c

and hold DOWN key. Text in buffer scrolls up but with jerks (random stops of
scrolling for a little time) due to fontification actions.

From 'jit-lock-defer-time' docs:

  Idle time after which deferred fontification should take place.
  If nil, fontification is not deferred.

I would try to workaround jerks with:

  (setq jit-lock-defer-time 0.01)

and with same negative effect by (see below):

  (setq jit-lock-defer-time 3.0)

When I hold DOWN key text scrolls up with jerks again! Because fontification
performed again.

I expect that when I *hold* key Emacs in not idle (it get user input and
redisplay text!!) so don't understand why fontification take place. May be I
misunderstand idling concept in Emacs and should read manuals deeper...

Also note that fixing CC-mode is not a solution as there are many possible
external modes that can be useful but with slow fontification.

So it is good to have 'jit-lock-defer-time' behaviour that allow trick with
defer fontification.

I know some peoples use workflow when they use scroll-bars for very fast
moving around a file (to get brief look of file content). I prefer use
PageUp/PageDown keys to perform this task.

In GNU Emacs 24.2.1 (x86_64-pc-linux-gnu, GTK+ Version 2.24.10)
 of 2012-09-09 on trouble, modified by Debian
Windowing system distributor `The X.Org Foundation', version 11.0.11204000

-- 
Best regards!