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Subject: bug#34038: 26.1; set-window-start sometimes fails to set window start
Resent-From: Markus Triska <triska@HIDDEN>
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From: Markus Triska <triska@HIDDEN>
Date: Thu, 10 Jan 2019 20:57:28 +0100
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To reproduce this issue, please start Emacs with "$ emacs -Q" and paste
at the end of the *scratch* buffer the following form:

    (progn
      (goto-char (point-max))
      (insert "\n")
      (dotimes (x (window-height))
        (insert (format "%s\n" x)))
      (redisplay)
      (let ((b (buffer-string))
            (p (point)))
        (set-window-start nil p t)
        (read-key "Please press a key to continue.")
        (erase-buffer)
        (insert b)
        (set-window-start nil p t)))

Then, put point at the end of the form, and evaluate it with C-x C-e.

Conceptually, this program:

     1) inserts several (i.e, window-height) lines into the buffer
     2) sets window point to p, which is point after (1)
     3) WAITS FOR A KEY PRESS (please press a key at this point)
     4) erases and restores the buffer contents
     5) sets window point to p, the same point as in (2)

However, in many invocations (please try it repeatedly if necessary),
the visible portion of the buffer unexpectedly differs between (3) and
after the whole procedure.

In other words, when Emacs waits for a key press, a different portion of
the buffer is visible compared to after you press a key, even though
window-start is set to the same position in both cases.

Is there a way to reliably set the window-start in such situations?

Thank you very much!
Markus

In GNU Emacs 26.1 (build 1, x86_64-pc-linux-gnu, X toolkit, Xaw scroll bars)
 of 2018-11-18 built on debian
Repository revision: 07f8f9bc5a51f5aa94eb099f3e15fbe0c20ea1ea
Windowing system distributor 'The X.Org Foundation', version 11.0.11902000
System Description:	Debian GNU/Linux 9.6 (stretch)

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From: help-debbugs@HIDDEN (GNU bug Tracking System)
To: Markus Triska <triska@HIDDEN>
Subject: bug#34038: Acknowledgement (26.1; set-window-start sometimes
 fails to set window start)
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Date: Thu, 10 Jan 2019 19:58:02 +0000

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Message sent to bug-gnu-emacs@HIDDEN:

Subject: bug#34038: 26.1; set-window-start sometimes fails to set window start
Resent-From: Eli Zaretskii <eliz@HIDDEN>
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To: Markus Triska <triska@HIDDEN>
Cc: 34038 <at> debbugs.gnu.org
Date: Fri, 11 Jan 2019 09:03:55 +0200
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From: Eli Zaretskii <eliz@HIDDEN>
In-reply-to: <m2ftu05lqv.fsf@HIDDEN> (message from Markus Triska on Thu, 
 10 Jan 2019 20:57:28 +0100)
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> From: Markus Triska <triska@HIDDEN>
> Date: Thu, 10 Jan 2019 20:57:28 +0100
> 
>      1) inserts several (i.e, window-height) lines into the buffer
>      2) sets window point to p, which is point after (1)
>      3) WAITS FOR A KEY PRESS (please press a key at this point)
>      4) erases and restores the buffer contents
>      5) sets window point to p, the same point as in (2)
> 
> However, in many invocations (please try it repeatedly if necessary),
> the visible portion of the buffer unexpectedly differs between (3) and
> after the whole procedure.
> 
> In other words, when Emacs waits for a key press, a different portion of
> the buffer is visible compared to after you press a key, even though
> window-start is set to the same position in both cases.
> 
> Is there a way to reliably set the window-start in such situations?

I think this happens because you call set-window-start with last
argument non-nil.  Doing that tells the display engine that the
window-start point is just a suggestion, not a hard requirement.

Message sent to bug-gnu-emacs@HIDDEN:

Subject: bug#34038: 26.1; set-window-start sometimes fails to set window start
Resent-From: Markus Triska <triska@HIDDEN>
Original-Sender: "Debbugs-submit" <debbugs-submit-bounces <at> debbugs.gnu.org>
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Resent-Date: Fri, 11 Jan 2019 12:21:02 +0000
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To: Eli Zaretskii <eliz@HIDDEN>
Cc: 34038 <at> debbugs.gnu.org
From: Markus Triska <triska@HIDDEN>
References: <m2ftu05lqv.fsf@HIDDEN> <83sgxzhe04.fsf@HIDDEN>
Date: Fri, 11 Jan 2019 13:20:33 +0100
In-Reply-To: <83sgxzhe04.fsf@HIDDEN> (Eli Zaretskii's message of "Fri, 11 Jan
 2019 09:03:55 +0200")
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Eli Zaretskii <eliz@HIDDEN> writes:

> I think this happens because you call set-window-start with last
> argument non-nil.  Doing that tells the display engine that the
> window-start point is just a suggestion, not a hard requirement.

In the documentation of set-window-start, the last argument is described
as:

    Optional third arg NOFORCE non-nil inhibits next redisplay from
    overriding motion of point in order to display at this exact start.

From this text, this seems to be precisely what I need: I want to retain
point at this exact place, and I only want to change the window start,
not the point. In my use case, if I set this argument to "nil", then I
get unexpected point motion. Is there a way to reliably set window point
while at the same time preventing motion of point? I tried adding
(redisplay) at strategic places in my code, and this seems to work
somewhat better then, though at the cost of causing display flickering.

Thank you and all the best!
Markus

Last modified: Fri, 11 Jan 2019 12:30:02 UTC

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