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]

[-- Attachment #1: Type: text/plain, Size: 1054 bytes --]

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

[-- Attachment #2: f1 --]
[-- Type: application/octet-stream, Size: 339 bytes --]

L1
L2
L3
L4
L5
L6
L7
L8
L9
L10
L11
L12
L13
L14
L15
L16
L17
L18
L19
L20
L21
L22
L23
L24
L25
L26
L27
L28
L29
L30
L31
L32
L33
L34
L35
L36
L37
L38
L39
L40
L41
L42
L43
L44
L45
L46
L47
L48
L49
L50
L51
L52
L53
L54
L55
L56
L57
L58
L59
L60
L61
L62
L63
L64
L65
L66
L67
L68
L69
L70
L71
L72
L73
L74
L75
L76
L77
L78
L79
L80
L81
L82
L83
L84
L85
L86
L87

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: 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.

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.