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

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

[Mario Lang]

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,
  ⡍⠁⠗⠊⠕

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

[Nicolas Richard]

Mario Lang <mlang@delysid.org> writes:
> 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?

I tried it in my GUI emacs (just to try chess.el, in fact) and noticed
very frequent recentering of my window, even when not in the *chess-ics*
buffer.

By tracing "recenter" I found that tabulated-list-print calls (recenter)
without checking what the selected window is. 

here's the patch I applied, perhaps you can try it out and see if it
fixes the problem for you.

diff --git a/lisp/emacs-lisp/tabulated-list.el b/lisp/emacs-lisp/tabulated-list.el
index d0d71dd..dd1166f 100644
--- a/lisp/emacs-lisp/tabulated-list.el
+++ b/lisp/emacs-lisp/tabulated-list.el
@@ -323,7 +323,9 @@ to the entry with the same ID element as the current line."
     (if saved-pt
 	(progn (goto-char saved-pt)
 	       (move-to-column saved-col)
-	       (recenter))
+	       (when (eq (window-buffer (selected-window))
+                         (current-buffer))
+                 (recenter)))
       (goto-char (point-min)))))
 
 (defun tabulated-list-print-entry (id cols)

-- 
Nico.

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

[Mario Lang]

Nicolas Richard <theonewiththeevillook@yahoo.fr> writes:

> Mario Lang <mlang@delysid.org> writes:
>> 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?
>
> I tried it in my GUI emacs (just to try chess.el, in fact) and noticed
> very frequent recentering of my window, even when not in the *chess-ics*
> buffer.
>
> By tracing "recenter" I found that tabulated-list-print calls (recenter)
> without checking what the selected window is. 
>
> here's the patch I applied, perhaps you can try it out and see if it
> fixes the problem for you.

Very good catch!  Most of the flickering is now gone.
I think this should be committed, if others are fine with it as well.

> diff --git a/lisp/emacs-lisp/tabulated-list.el b/lisp/emacs-lisp/tabulated-list.el
> index d0d71dd..dd1166f 100644
> --- a/lisp/emacs-lisp/tabulated-list.el
> +++ b/lisp/emacs-lisp/tabulated-list.el
> @@ -323,7 +323,9 @@ to the entry with the same ID element as the current line."
>      (if saved-pt
>  	(progn (goto-char saved-pt)
>  	       (move-to-column saved-col)
> -	       (recenter))
> +	       (when (eq (window-buffer (selected-window))
> +                         (current-buffer))
> +                 (recenter)))
>        (goto-char (point-min)))))
>
>  (defun tabulated-list-print-entry (id cols)

-- 
CYa,
  ⡍⠁⠗⠊⠕

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

[Nicolas Richard]

Mario Lang <mlang@delysid.org> writes:
>>> 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.

Perhaps you should file a bug report so as to ensure this does not get
forgotten. FWIW, I would even completely remove the call to (recenter)
in tabulated-list-print because I don't know what purpose it serves.

-- 
Nico.

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

[Stefan Monnier]

> -	       (recenter))
> +	       (when (eq (window-buffer (selected-window))
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Aka                               (window-buffer)

> +                         (current-buffer))
> +                 (recenter)))

Looks like you found a bug here indeed.  FWIW, we fixed a similar bug
a while ago elsewhere.  I think maybe `recenter' should signal an error
if (window-buffer) is different from (current-buffer) to help us catch
those problem.


        Stefan