bug#24179: 25.1; scroll-conservatively over SCROLL_LIMIT may put point in the wrong place

[Alex]

This bug report originated from
https://github.com/magit/magit/issues/2735

If scroll-conservatively is higher than SCROLL_LIMIT (100), then trying
to move down past an overlay with the `before-string' property may put
the point at the top of the window rather than at the next available
point. This only occurs when the overlay's line is only partially
visible.

This can be reproduced without magit, but it's easiest to reproduce with
magit:

1. Start in emacs -Q. and load magit.
2. Go to version.el in emacs' repo
3. Set scroll-conservatively to a value higher than 100.
4. magit-blame.
5. M-g c 1350
6. Press C-f

The point is now at 1351, but now it's at the top of the screen.

The above should work if the screen is maximized on a 1080p screen.
Otherwise you can just keep hitting `n' until you reach a
partially-visible overlay.

In GNU Emacs 25.1.1 (x86_64-redhat-linux-gnu, GTK+ Version 3.20.6)
 of 2016-07-25 built on buildhw-08.phx2.fedoraproject.org
Windowing system distributor 'Fedora Project', version 11.0.11804000
Configured using:
 'configure --build=x86_64-redhat-linux-gnu
 --host=x86_64-redhat-linux-gnu --program-prefix=
 --disable-dependency-tracking --prefix=/usr --exec-prefix=/usr
 --bindir=/usr/bin --sbindir=/usr/sbin --sysconfdir=/etc
 --datadir=/usr/share --includedir=/usr/include --libdir=/usr/lib64
 --libexecdir=/usr/libexec --localstatedir=/var
 --sharedstatedir=/var/lib --mandir=/usr/share/man
 --infodir=/usr/share/info --with-dbus --with-gif --with-jpeg --with-png
 --with-rsvg --with-tiff --with-xft --with-xpm --with-x-toolkit=gtk3
 --with-gpm=no --with-xwidgets build_alias=x86_64-redhat-linux-gnu
 host_alias=x86_64-redhat-linux-gnu 'CFLAGS=-DMAIL_USE_LOCKF -O2 -g
 -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2
 -fexceptions -fstack-protector-strong --param=ssp-buffer-size=4
 -grecord-gcc-switches -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1
 -m64 -mtune=generic' LDFLAGS=-Wl,-z,relro
 PKG_CONFIG_PATH=:/usr/lib64/pkgconfig:/usr/share/pkgconfig'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND DBUS GCONF GSETTINGS NOTIFY
ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 XWIDGETS

Major mode: Emacs-Lisp

Minor modes in effect:
  magit-auto-revert-mode: t
  auto-revert-mode: t
  global-git-commit-mode: t
  async-bytecomp-package-mode: t
  shell-dirtrack-mode: t
  diff-auto-refine-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-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

Features:
(shadow sort mail-extr emacsbug sendmail linum magit-blame magit-stash
magit-bisect magit-remote magit-commit magit-sequence magit magit-apply
magit-wip magit-log magit-diff smerge-mode magit-core magit-autorevert
autorevert filenotify magit-process magit-popup magit-mode magit-git crm
magit-section magit-utils git-commit log-edit message dired rfc822 mml
mml-sec epg mm-decode mm-bodies mm-encode mail-parse rfc2231 rfc2047
rfc2045 ietf-drums mailabbrev mail-utils gmm-utils mailheader pcvs-util
with-editor async-bytecomp async tramp-sh tramp tramp-compat auth-source
cl-seq eieio eieio-core cl-macs gnus-util mm-util help-fns mail-prsvr
password-cache tramp-loaddefs trampver ucs-normalize shell pcomplete
comint ansi-color ring format-spec advice server dash vc-git diff-mode
easy-mmode bug-reference add-log finder-inf tex-site slime-autoloads
info package epg-config seq byte-opt gv bytecomp byte-compile cl-extra
help-mode easymenu cconv cl-loaddefs pcase cl-lib time-date mule-util
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel x-win term/common-win x-dnd tool-bar dnd fontset image regexp-opt
fringe tabulated-list newcomment elisp-mode lisp-mode prog-mode register
page menu-bar rfn-eshadow timer select scroll-bar mouse jit-lock
font-lock syntax facemenu font-core frame cl-generic cham georgian
utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao korean
japanese eucjp-ms cp51932 hebrew greek romanian slovak czech european
ethiopic indian cyrillic chinese charscript case-table epa-hook
jka-cmpr-hook help simple abbrev minibuffer cl-preloaded nadvice
loaddefs button faces cus-face macroexp files text-properties overlay
sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote dbusbind inotify dynamic-setting
system-font-setting font-render-setting xwidget-internal move-toolbar
gtk x-toolkit x multi-tty make-network-process emacs)