GNU bug report logs - #13248
24.2.50; select-window moves point (in comint-postoutput-scroll-to-bottom)

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From: martin rudalics <rudalics <at> gmx.at>
To: Vitalie Spinu <spinuvit <at> gmail.com>
Cc: 13248 <at> debbugs.gnu.org
Subject: bug#13248: 24.2.50; select-window moves point (in comint-postoutput-scroll-to-bottom)
Date: Fri, 21 Dec 2012 15:25:07 +0100

> Here is what I found during my investigation.  It happens with
> (select-window w) in the `comint-postoutput-scroll-to-bottom'
> (reproduced below). I was monitoring the value of the (point) just
> before and after it, and it looks like this:
>
>  before (point):9943
>  after  (point):8619
>
> So the point is clearly moved in select-window. Moreover the (point)
> equals (window-end) just before select-window is called, so it is
> visible. Consequently, the following (comint-adjust-point selected) is
> completely screwed because it relies on point *not* being moved!
>
> Here is a relevant piece of `comint-postoutput-scroll-to-bottom'
>
>
> ╭──────── #2124 ─ /home/vitoshka/TVC/emacs/lisp/comint.el ──
> │ 	    (dolist (w (get-buffer-window-list current nil t))
> │ 	      (select-window w)
> │ 	      (unwind-protect
> │ 		  (progn
> │ 		    (comint-adjust-point selected)
> │ 		    ;; Optionally scroll to the bottom of the window.
> │ 		    (and comint-scroll-show-maximum-output
> │ 			 (eobp)
> │ 			 (recenter (- -1 scroll-margin))))
> │ 		(select-window selected))))))
> │       (set-buffer current))))
> ╰──────── #2134 ─
>
>
> The variable `comint-scroll-show-maximum-output' is the default t, this
> is why comint runs `comint-postoutput-scroll-to-bottom' in its
> `comint-output-filter-functions'.
>
> I can propose a patch for commit to reset the point, but it looks like
> there is something much more fundamental going on in here.

select_window (in window.c) has the following comment

  /* Go to the point recorded in the window.
     This is important when the buffer is in more
     than one window.  It also matters when
     redisplay_window has altered point after scrolling,
     because it makes the change only in the window.  */

Is it this behavior that bothers you?

IIUC what you want in `comint-postoutput-scroll-to-bottom' is to
`set-window-point' of the respective window.  If you really want to move
`point' in a buffer _and_ show the effect in a window, do it with that
window selected.

martin

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