GNU bug report logs - #148
[Bernardo Dal Seno]: scroll-margin and scroll-preserve-screen-position cause refresh problem

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Message #5 received at submit <at> emacsbugs.donarmstrong.com (full text, mbox):

From: Sven Joachim <svenjoac <at> gmx.de>
To: submit <at> debbugs.gnu.org
Subject: [Bernardo Dal Seno]: scroll-margin and scroll-preserve-screen-position cause refresh problem
Date: Sat, 19 Apr 2008 09:14:00 +0200

[Message part 1 (text/plain, inline)]

Package: emacs
Version: 22.1, 22.2.50, 23.0.60

I received the attached message via the Debian bug tracking system.  The
described redisplay problem is reproducible in both EMACS_22_BASE and
the trunk.  While the scrollbar and the modeline are updated upon C-v,
the buffer window is not.

[Message part 2 (message/rfc822, inline)]

From: Bernardo Dal Seno <dibbex <at> gmail.com>
To: Debian Bug Tracking System <submit <at> bugs.debian.org>
Subject: Bug#470556: emacs22: scroll-margin and scroll-preserve-screen-position
 cause refresh problem
Date: Tue, 11 Mar 2008 22:34:29 +0100

Package: emacs22
Version: 22.1+1-3
Severity: normal

With this very minimal '.emacs' file

---BEGIN---
(custom-set-variables
 '(scroll-margin 2)
 '(scroll-preserve-screen-position t)
) 
---END--- 

hitting page-up and page-down (or C-v and M-v) in any order just after 
having open an exiting file does not update the buffer view immediatly. 

In other words, you open a file, and you hit page-down a couple of 
times; the scroll bar and the status bar get updated immediatly, while 
the content of the window is update only after some time, which could be 
less than a second (but the delay is noticeable) or also a few seconds.

If you move the cursor using the arrow keys, the problem disappear.  If 
you move the cursor to the very first line of the buffer, though, and 
then hit page-down, you'll see the problem again.

I'm using X.  In a terminal window Emacs seems to work fine.

I've experienced this problem for a while, but only now I found the 
cause.  It's more apparent with files of a few hundred lines.


Best regards,
Bernardo

-- System Information:
Debian Release: lenny/sid
  APT prefers unstable
  APT policy: (500, 'unstable'), (1, 'experimental')
Architecture: i386 (i686)

Kernel: Linux 2.6.23 (PREEMPT)
Locale: LANG=en_US.utf8, LC_CTYPE=en_US.utf8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/bash

Versions of packages emacs22 depends on:
ii  emacs22-bin-common     22.1+1-3          The GNU Emacs editor's shared, arc
ii  libasound2             1.0.16-1          ALSA library
ii  libc6                  2.7-8             GNU C Library: Shared libraries
ii  libice6                2:1.0.4-1         X11 Inter-Client Exchange library
ii  libjpeg62              6b-14             The Independent JPEG Group's JPEG 
ii  libncurses5            5.6+20080203-1    Shared libraries for terminal hand
ii  libpng12-0             1.2.15~beta5-3    PNG library - runtime
ii  libsm6                 2:1.0.3-1+b1      X11 Session Management library
ii  libtiff4               3.8.2-7           Tag Image File Format (TIFF) libra
ii  libx11-6               2:1.0.3-7         X11 client-side library
ii  libxext6               2:1.0.4-1         X11 miscellaneous extension librar
ii  libxmu6                2:1.0.4-1         X11 miscellaneous utility library
ii  libxpm4                1:3.5.7-1         X11 pixmap library
ii  libxt6                 1:1.0.5-3         X11 toolkit intrinsics library
ii  xaw3dg                 1.5+E-15          Xaw3d widget set
ii  zlib1g                 1:1.2.3.3.dfsg-11 compression library - runtime

emacs22 recommends no packages.

-- no debconf information

Message #10 received at 148 <at> emacsbugs.donarmstrong.com (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: Sven Joachim <svenjoac <at> gmx.de>
Cc: 148 <at> debbugs.gnu.org
Subject: Re: bug#148: [Bernardo Dal Seno]: scroll-margin and scroll-preserve-screen-position cause refresh problem
Date: Sat, 19 Apr 2008 10:20:51 -0400

> I received the attached message via the Debian bug tracking system.  The
> described redisplay problem is reproducible in both EMACS_22_BASE and
> the trunk.  While the scrollbar and the modeline are updated upon C-v,
> the buffer window is not.

This behavior is not a bug in itself, it's just a symptom: it means that
redisplay was interrupted by user input before it had time
to terminate.  See redisplay-dont-pause.

So, the problem you're reporting is that Emacs's redisplay is too slow
to keep up with the scrolling (which is indeed a problem).  This may of
course depend on the file's content and the major-mode (since part of the
redisplay time might be used by font-locking, for example).


        Stefan

Message #15 received at 148 <at> emacsbugs.donarmstrong.com (full text, mbox):

From: Sven Joachim <svenjoac <at> gmx.de>
To: Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: 148 <at> debbugs.gnu.org
Subject: Re: bug#148: [Bernardo Dal Seno]: scroll-margin and scroll-preserve-screen-position cause refresh problem
Date: Sun, 20 Apr 2008 15:57:18 +0200

On 2008-04-19 16:20 +0200, Stefan Monnier wrote:

> This behavior is not a bug in itself, it's just a symptom: it means that
> redisplay was interrupted by user input before it had time
> to terminate.  See redisplay-dont-pause.

Okay, maybe the submitter of the Debian bug should set
redisplay-dont-pause to t to work around the problem.

> So, the problem you're reporting is that Emacs's redisplay is too slow
> to keep up with the scrolling (which is indeed a problem).  This may of
> course depend on the file's content and the major-mode (since part of the
> redisplay time might be used by font-locking, for example).

Do you have an explanation why the combination of

(setq scroll-margin 2)
(setq scroll-preserve-screen-position)

slows down redisplay so much on C-v, especially if point is in the
middle of the window?  I've never seen the problem in the default
settings.

Sven

Message #20 received at 148 <at> emacsbugs.donarmstrong.com (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: Sven Joachim <svenjoac <at> gmx.de>
Cc: 148 <at> debbugs.gnu.org
Subject: Re: bug#148: [Bernardo Dal Seno]: scroll-margin and scroll-preserve-screen-position cause refresh problem
Date: Mon, 21 Apr 2008 07:44:46 -0400

>> This behavior is not a bug in itself, it's just a symptom: it means that
>> redisplay was interrupted by user input before it had time
>> to terminate.  See redisplay-dont-pause.

> Okay, maybe the submitter of the Debian bug should set
> redisplay-dont-pause to t to work around the problem.

Yes, that should work around the problem.

>> So, the problem you're reporting is that Emacs's redisplay is too slow
>> to keep up with the scrolling (which is indeed a problem).  This may of
>> course depend on the file's content and the major-mode (since part of the
>> redisplay time might be used by font-locking, for example).

> Do you have an explanation why the combination of

> (setq scroll-margin 2)
> (setq scroll-preserve-screen-position)

> slows down redisplay so much on C-v, especially if point is in the
> middle of the window?  I've never seen the problem in the default
> settings.

I do not have an explanation, no.


        Stefan

Message #25 received at 148 <at> emacsbugs.donarmstrong.com (full text, mbox):

From: "Bernardo Dal Seno" <dibbex <at> gmail.com>
To: "Stefan Monnier" <monnier <at> iro.umontreal.ca>
Cc: 148 <at> debbugs.gnu.org
Subject: Re: bug#148: [Bernardo Dal Seno]: scroll-margin and scroll-preserve-screen-position cause refresh problem
Date: Tue, 22 Apr 2008 16:53:58 +0200

> This behavior is not a bug in itself, it's just a symptom: it means that
> redisplay was interrupted by user input before it had time
> to terminate.  See redisplay-dont-pause.

I tried the following .emacs:
---BEGIN---
(custom-set-variables
 '(scroll-margin 2)
 '(scroll-preserve-screen-position t)
 '(redisplay-dont-pause t)
)
---END---
and the bug appeared as before.


> This may of
> course depend on the file's content and the major-mode (since part of the
> redisplay time might be used by font-locking, for example).

In order to avoid font-locking and strange hooks, I copied a plain
text file to '/tmp/xxyyzz' and opened it with Emacs.  Same problem.

Actually, the display isn't updated if I scroll the buffer when the
cursor is in the first two lines of the window.  A partial work-around
I've found is adding this to .emacs:
---CUT---
(add-hook 'find-file-hook
    (function (lambda ()
                      (goto-line (+ 1 scroll-margin))
                  )))
---CUT---
When I open a file I can scroll it immediately, as the cursor is on
the third line, but whenever I move the cursor to the beginning of the
buffer, the bug shows up again.

I don't know anything about Emacs internals, but it seems a very
strange behavior.


Bernardo

Message #30 received at 148 <at> emacsbugs.donarmstrong.com (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: "Bernardo Dal Seno" <dibbex <at> gmail.com>
Cc: 148 <at> debbugs.gnu.org
Subject: Re: bug#148: [Bernardo Dal Seno]: scroll-margin and scroll-preserve-screen-position cause refresh problem
Date: Tue, 22 Apr 2008 13:05:07 -0400

>> This behavior is not a bug in itself, it's just a symptom: it means that
>> redisplay was interrupted by user input before it had time
>> to terminate.  See redisplay-dont-pause.

> I tried the following .emacs:
> ---BEGIN---
> (custom-set-variables
>  '(scroll-margin 2)
>  '(scroll-preserve-screen-position t)
>  '(redisplay-dont-pause t)
> )
> ---END---
> and the bug appeared as before.

> Actually, the display isn't updated if I scroll the buffer when the
> cursor is in the first two lines of the window.  A partial work-around

Oh, indeed, I see something very odd going on, now.  It's not just the
redisplay-dont-pause, it's a real bug: it looks like if the cursor starts
inside the `scroll-margin' (e.g. (point-min)) and we scroll a page
forward, the cursor is moved ans so is point, but the text is
not updated.


        Stefan

Message #35 received at 148-done <at> emacsbugs.donarmstrong.com (full text, mbox):

From: Chong Yidong <cyd <at> stupidchicken.com>
To: Sven Joachim <svenjoac <at> gmx.de>
Cc: 148-done <at> debbugs.gnu.org
Subject: Re: [Bernardo Dal Seno]: scroll-margin and scroll-preserve-screen-position cause refresh problem
Date: Tue, 05 Aug 2008 18:48:08 -0400

> With this very minimal '.emacs' file
>
> (custom-set-variables
>  '(scroll-margin 2)
>  '(scroll-preserve-screen-position t)) 
>
> hitting page-up and page-down (or C-v and M-v) in any order just after 
> having open an exiting file does not update the buffer view immediatly. 

I've just checked in a fix.  Thanks.

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