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

From: Eric M. Ludlam
Subject: bug#15045: Point jumps inappropriately around time of Semantic lexing
Date: Fri, 09 Aug 2013 07:50:19 -0400
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.3a1pre) Gecko/20091222 Shredder/3.1a1pre

On 08/09/2013 03:53 AM, Eli Zaretskii wrote:
AFAICS, any function that calls things like
`accept-process-output', `input-pending-p' or `sit-for' inside a
`save-excursion' might get an unwanted scrolling effect if point is
temporarily moved to some invisible location.

You mean, because of timers that might get run and cause redisplay?
Yes, that's possible.  But the way to avoid this is to never call
these when point is in a place the user won't expect.  Forcing the
Emacs community not to trigger redisplay inside timers is IMO _not_
the right way, because this is unnecessarily restrictive, and would
disallow a whole bunch of useful features for no good reason.

IOW, it's the caller of input-pending-p etc. that is the fault here,
not the timers that it inadvertently lets run.

In my mind, to anyone using input-pending-p to deal with responsiveness in long running code, a timer that causes a redisplay is the source of the problem. In addition, it is clear that anyone with a sit-for in their timer will think that moving the point and calling input-pending-p is a bug.

Even if we resolve which is the real bug, or at least the proposed solution here, people will still cause redisplays in timeres, and call input-pending-p in long running code, and those problems will need to be debugged again in the future.

It seems like it would be more developer friendly for Emacs to enforce some kind of behavior with timers so as to remove this class of collision. Here are some random options:

* In a timer, disallow dispatch of other timers until done.
* Record redisplay requests in timers, and force redisplay when the
  timer exists.
* In input-pending-p, dont' allow new timers to run,
  doc only talks about command input, not timers.
* In timers, redisplay always assumes you don't want to move point and
  puts it back before the draw.

It is clear to me that there are valid use cases for both redisplay in a timer, and input-pending-p being used in long-running programs. Adding funky restrictions to either seems untenable with the size of the Emacs dev community. It would be better to tackle the problem at its source.


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