bug#14508: scroll-conservatively==1 not honored for fast line by line navigation up

bug#14508: scroll-conservatively==1 not honored for fast line by line navigation up

[Barry OReilly]

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Let /tmp/scroll-conservatively-1.el have this content:

  (setq scroll-conservatively 1)
  (add-hook 'term-setup-hook (lambda () (goto-char (point-max))))

And /tmp/foo.cpp as attached.

Also set the keyboard repitition rate somewhat high.

Start Emacs by: emacs -Q /tmp/foo.cpp --load /tmp/scroll-conservatively-1.el

Hold arrow up key, recentering occurs while scrolling up.

Note: Script goes to EOB because I only witness this for scroll ups, not
scroll downs.

I witness this regularly when navigating C++ source code.

----

In GNU Emacs 24.3.50.6 (x86_64-unknown-linux-gnu, GTK+ Version 2.10.4)
 of 2013-05-23 on redacted
Windowing system distributor `The X.Org Foundation', version 11.0.70101000
System Description:    Red Hat Enterprise Linux Client release 5.4 (Tikanga)

Configured using:
 `configure --prefix=/redacted/user/boreilly/sw/emacs-install-trunk
 --with-gif=no'

Important settings:
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=none
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: C++/l

Minor modes in effect:
  tooltip-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t
  abbrev-mode: t

Recent input:
M-x r e p o r t - e m a c <tab> <return>

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
foo.cpp has auto save data; consider M-x recover-this-file
Loading cc-langs...done

Load-path shadows:
None found.

Features:
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env code-pages mule custom widget hashtable-print-readable backquote
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font-render-setting move-toolbar gtk x-toolkit x multi-tty emacs)

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bug#14508: scroll-conservatively==1 not honored for fast line by line navigation up

[Eli Zaretskii]

> Date: Wed, 29 May 2013 18:58:33 -0400
> From: Barry OReilly <gundaetiapo@gmail.com>
> 
> Let /tmp/scroll-conservatively-1.el have this content:
> 
>   (setq scroll-conservatively 1)
>   (add-hook 'term-setup-hook (lambda () (goto-char (point-max))))
> 
> And /tmp/foo.cpp as attached.
> 
> Also set the keyboard repitition rate somewhat high.
> 
> Start Emacs by: emacs -Q /tmp/foo.cpp --load /tmp/scroll-conservatively-1.el
> 
> Hold arrow up key, recentering occurs while scrolling up.
> 
> Note: Script goes to EOB because I only witness this for scroll ups, not
> scroll downs.
> 
> I witness this regularly when navigating C++ source code.

This is normal.  Sometimes, Emacs redisplay takes more time, for
various reasons (e.g., garbage collection triggered by JIT font-lock
that fontifies portions of text that come into view).  When redisplay
finishes, Emacs finds more than a single keystroke in its keyboard
queue, and processes all of them, which moves point more than 1 line
out of the window.  Since scroll-conservatively is set to 1, Emacs
recenters.

You see this on scroll ups, because they are slightly slower than
scroll downs, and the difference is enough to trigger this with higher
probability when you go up.

To avoid this, set scroll-conservatively to higher values.  For
example, I just set it to 3 and was able to scroll through the whole
file you sent without even a single recenter.

bug#14508: scroll-conservatively==1 not honored for fast line by line navigation up

[Barry OReilly]

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> You see this on scroll ups, because they are slightly slower than
> scroll downs, and the difference is enough to trigger this with
> higher probability when you go up.

Closely related, I define these commands (byte compiled):

  (defvar my-leap-scroll-size 16)
  (define-key evil-normal-state-map ";" nil)
  (define-key evil-motion-state-map ";"
    (lambda ()
      (interactive)
      (scroll-up my-leap-scroll-size)
      (evil-next-line my-leap-scroll-size)))
  (define-key evil-normal-state-map "'" nil)
  (define-key evil-motion-state-map "'"
    (lambda ()
      (interactive)
      (scroll-down my-leap-scroll-size)
      (evil-previous-line my-leap-scroll-size)))

In the same C++ buffer as before, when I hold to repeat the above
command that scrolls up*, the display doesn't update at all until I
release the key to stop the repeat. As you indicated it may, the
scroll down can keep up in this case. I turned font locking off and
the scroll up redisplay keeps up. I also tried this in a Python buffer
with font lock on and these commands keep up much better. Is it that
the C/C++ font locking is more complex? Has C/C++ major mode been
performance tuned for this use case? Maybe I can experiment with my
local configuration to temporarily disable font locking while I'm
scrolling up fast -- seeing non font locked code fly by would be
better than nothing at all.

* Assume I use "up" and "down" in the majority usage, opposite of
  Emacs terminology

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bug#14508: scroll-conservatively==1 not honored for fast line by line navigation up

[Eli Zaretskii]

> Date: Fri, 7 Jun 2013 12:37:08 -0400
> From: Barry OReilly <gundaetiapo@gmail.com>
> Cc: 14508@debbugs.gnu.org
> 
> In the same C++ buffer as before, when I hold to repeat the above
> command that scrolls up*, the display doesn't update at all until I
> release the key to stop the repeat. As you indicated it may, the
> scroll down can keep up in this case. I turned font locking off and
> the scroll up redisplay keeps up. I also tried this in a Python buffer
> with font lock on and these commands keep up much better. Is it that
> the C/C++ font locking is more complex?

Could be, I wouldn't know (and don't use Python enough to share my
experience).

> Has C/C++ major mode been
> performance tuned for this use case? Maybe I can experiment with my
> local configuration to temporarily disable font locking while I'm
> scrolling up fast -- seeing non font locked code fly by would be
> better than nothing at all.

I'd suggest to try activating jit-lock-stealth and/or jit-lock-defer
instead.  These two features should make the problem less acute, once
you tune the customization parameters, jit-lock-stealth-time and
jit-lock-defer-time, correspondingly.

bug#14508: scroll-conservatively==1 not honored for fast line by line navigation up

[Barry OReilly]

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I ended up settling on settings:

 '(font-lock-maximum-decoration (quote ((c++-mode . 2))))
 '(scroll-conservatively 101)
 '(scroll-margin 4)

and in the C++ mode hook:

  (setq jit-lock-defer-time 0.01)

So I am able to scroll through C++ buffers with timely redisplays and
without recenterings. Closing.

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