comint-preoutput-filter-functions and flickering redisplay in TTY

[Mario Lang <mlang@delysid.org>]

Hi.

chess.el can talk to internet chess servers.  This is based on comint.
ICS periodically announces "seeking game" messages which we collect
into a separate buffer, as there are potentially many of them, which
would totally clutter your comint output.  For this, we use
comint-preoutput-filter-functions to catch the output before it goes to
the buffer.  If the string arriving in comint matches, we insert it
somewhere else, and return "".  Now, I am noticing strange redisplay
behaviour if "seeking game" messages are on in Emacs -nw.
There is noticeable flickering of the whole screen.
As if the screen was cleared completely, and redrawn again from scratch.
But there are no big visual changes going on actually, the screen
content stays largely the same.

I am out of ideas on this one.  Does anyone have ideas why this might be
happening, and if there is some sort of work-around I could use from
Lisp-world to avoid it?

To reproduce: Install chess.el, call M-x chess-ics RET, login to
freechess.org (as guest is OK), and wait a bit.  Your screen (at least in -nw mode)
should start to flicker noticeably every once in a while.

Any ideas why?

-- 
Thanks,
  ⡍⠁⠗⠊⠕