bug#21469: 25.0.50; right scrollbar wrongly placed when resizing split window

[Robert Pluim <rpluim@gmail.com>]

To reproduce:

emacs -Q
C-x 3 # split-window-right
Then use the mouse to move the horizontal separator in the middle of
the mode-line to the left. The frame stays the same size, but the
right scrollbar moves inwards from the right edge. It also appears
that the right scrollbar controls the window to the left, not the
window to the right.

This is with git emacs commit 166812addb0a1b9b2258ef3ce76424cd7facfa8b

If I rebuild using ‘--without-toolkit-scroll-bars’ everything works as
expected.

In GNU Emacs 25.0.50.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.14.13)
 of 2015-09-13
Windowing system distributor 'The X.Org Foundation', version 11.0.11701000
System Description:	Ubuntu 15.04

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

Important settings:
  value of $LC_COLLATE: C
  value of $LC_MONETARY: en_GB.UTF-8
  value of $LC_NUMERIC: en_GB.UTF-8
  value of $LC_TIME: en_GB.UTF-8
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Group

Minor modes in effect:
  gnus-topic-mode: t
  gnus-undo-mode: t
  diff-auto-refine-mode: t
  desktop-save-mode: t
  show-paren-mode: t
  savehist-mode: t
  ido-everywhere: t
  global-auto-revert-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  buffer-read-only: t
  size-indication-mode: t
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent messages:
Decrypting /home/rpluim/.authinfo.gpg...done
Reading /home/rpluim/.newsrc.eld...
Opening nntp server on eternal...done
Opening nnimap server on GMail...done
Opening nnimap server on Fortinet...done
No new newsgroups
Checking new news...
Reading active file via nndraft...done
Checking new news...done
Making completion list... [2 times]


Features:
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iso-transl face-remap key-chord dired+ image-dired image-file dired-x
dired-aux dired desktop frameset ox-ioslide f dash s ucs-normalize
ox-odt rng-loc rng-uri rng-parse rng-match rng-dt rng-util rng-pttrn
nxml-parse nxml-ns nxml-enc xmltok nxml-util ox-html ox-beamer
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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
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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
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dynamic-setting system-font-setting font-render-setting move-toolbar
gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 561175 17840)
 (symbols 48 135516 0)
 (miscs 40 548 318)
 (strings 32 149055 20453)
 (string-bytes 1 4797262)
 (vectors 16 62885)
 (vector-slots 8 1059839 6781)
 (floats 8 514 264)
 (intervals 56 659 61)
 (buffers 976 33)
 (heap 1024 114218 2399))