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

[Eli Zaretskii]

> From: Robert Pluim <rpluim@gmail.com>
> Date: Sun, 13 Sep 2015 15:31:34 +0200
> 
> 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.

Doesn't happen on w32 with the current master, FWIW.  Probably
GTK-specific.