bug#15045: Point jumps inappropriately around time of Semantic lexing

[Barry OReilly <gundaetiapo@gmail.com>]

>> Undesired scrolling is a downstream symptom. Upstream from it is
>> point visibly moving around inappropriately.

> I think you are wrong. Emacs moves point in its Lisp code all over
> the place, and that never causes any unwarranted scrolling, nor
> should it ever display point in anything but the final position --
> unless the Lisp code itself forces redisplay.

My use of the adverb "visibly" was meant to indicate redisplay of
point at other positions. Sorry to have confused you.

>   timer-event-handler([t 20995 27860 0 60 display-time-event-handler nil nil 0])
>   accept-process-output()
>   semantic-c-lexer(2886 7946 nil nil)

and

>   timer-event-handler([t 20995 36800 0 60 display-time-event-handler nil nil 0])
>   input-pending-p()
>   semantic-c-lexer(2939 7944 nil nil)

This is indeed a key finding.

If arbitrary timers can execute during the lexer's call to
accept-process-output or input-pending-p, then doesn't that mean
jit-lock-deferred-fontify can run too? If removing timers' sit-for
calls is the solution, then what's to become of
jit-lock-deferred-fontify's call to sit-for? Doesn't deferred jit
locking necessarily have to call redisplay?