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

[Dani Moncayo]

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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