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

[martin rudalics <rudalics@gmx.at>]

 > 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

I cannot reproduce that here with GTK+ 3.4.2.  Does it also happen when
you do (set-frame-parameter nil 'right-divider-width 6)?  Could this be
cairo related?

martin