bug#22395: 25.0.50; 'C-v' scrolls only a half of the window height

bug#22395: 25.0.50; 'C-v' scrolls only a half of the window height

[Dani Moncayo]

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Recipe from "emacs -Q":

1. Eval:
    (setq scroll-preserve-screen-position t)
    (setq scroll-margin 2)

2. Visit the attached file.

On my system, I initially see lines 1 to 33.

3. Type 'C-v'

I observe that 'C-v' has scrolled the buffer vertically, but only by
about a half of the window height (I now see lines 16 to 48).
Subsequent 'C-v' commands do the same: they scroll the buffer by a
half of the window.

This seems wrong.  The amount of scrolling should be "a near full
screen", as described in 'C-h k C-v'.

I observe this bug also in the last 24.5 release.

In GNU Emacs 25.0.50.1 (i686-pc-mingw32)
 of 2016-01-17 built on LEG570
Repository revision: a0d5b7ae3bb014bb0b1c205d123c597df0e76411
Windowing system distributor 'Microsoft Corp.', version 10.0.10586
Configured using:
 'configure --host=i686-pc-mingw32 --enable-checking=yes,glyphs'

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND NOTIFY ACL GNUTLS LIBXML2 ZLIB
TOOLKIT_SCROLL_BARS

Important settings:
  value of $LANG: ENU
  locale-coding-system: cp1252

-- 
Dani Moncayo

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bug#22395: 25.0.50; 'C-v' scrolls only a half of the window height

[martin rudalics]

 > Recipe from "emacs -Q":
 >
 > 1. Eval:
 >      (setq scroll-preserve-screen-position t)
 >      (setq scroll-margin 2)
 >
 > 2. Visit the attached file.
 >
 > On my system, I initially see lines 1 to 33.
 >
 > 3. Type 'C-v'
 >
 > I observe that 'C-v' has scrolled the buffer vertically, but only by
 > about a half of the window height (I now see lines 16 to 48).
 > Subsequent 'C-v' commands do the same: they scroll the buffer by a
 > half of the window.
 >
 > This seems wrong.  The amount of scrolling should be "a near full
 > screen", as described in 'C-h k C-v'.
 >
 > I observe this bug also in the last 24.5 release.

I'm not sure whether it's a bug or a feature.  But here I can "fix" your
behavior by customizing ‘scroll-conservatively’ to 101.

martin

bug#22395: 25.0.50; 'C-v' scrolls only a half of the window height

[Dani Moncayo]

> I'm not sure whether it's a bug or a feature.

I fail to see how this could be a feature.

> But here I can "fix" your
> behavior by customizing ‘scroll-conservatively’ to 101.

That would not solve the underlying problem, and some users (like
myself) may prefer another value for 'scroll-conservatively'.

-- 
Dani Moncayo

bug#22395: 25.0.50; 'C-v' scrolls only a half of the window height

[Eli Zaretskii]

> Date: Sun, 17 Jan 2016 21:50:16 +0100
> From: Dani Moncayo <dmoncayo@gmail.com>
> 
> 1. Eval:
>     (setq scroll-preserve-screen-position t)
>     (setq scroll-margin 2)
> 
> 2. Visit the attached file.
> 
> On my system, I initially see lines 1 to 33.
> 
> 3. Type 'C-v'
> 
> I observe that 'C-v' has scrolled the buffer vertically, but only by
> about a half of the window height (I now see lines 16 to 48).
> Subsequent 'C-v' commands do the same: they scroll the buffer by a
> half of the window.
> 
> This seems wrong.  The amount of scrolling should be "a near full
> screen", as described in 'C-h k C-v'.

Start the same recipe from line #3 or greater, and the "bug" goes
away.

You asked Emacs to do the impossible: preserve the screen position of
point at the first window line and also not to allow point to enter
the first 2 lines of the window.  Emacs didn't know what to do with
this conflict.  Now it does.

bug#22395: 25.0.50; 'C-v' scrolls only a half of the window height

[Eli Zaretskii]

> Date: Mon, 18 Jan 2016 11:30:50 +0100
> From: martin rudalics <rudalics@gmx.at>
> 
> I'm not sure whether it's a bug or a feature.

It was a conflict between 2 features.

> But here I can "fix" your behavior by customizing
> ‘scroll-conservatively’ to 101.

Yes, because scroll-conservatively overrides
scroll-preserve-screen-position.