bug#23690: 25.0.94; Window resizing regression

[Stephen Berman]

0. emacs -Q
1. Type `C-x 3' to split the selected window into two side-by-side
   windows.
2. Sanity check: click mouse-1 on the divider between the mode lines,
   and observe that by dragging the mouse you can move the vertical
   boundary between the windows to the left or right.
3. In the menu Options->Show/Hide->Scroll-bar click the "On the Left"
   button to make the scroll bars appear on the left side of the
   windows.
4. Repeat step 2.
=> The vertical boundary between the windows does not change, but
   instead the echo area displays the message "No window on the right of
   this one" (although the selected window is the one on the left).

This is a regression from Emacs 24.5, where step 4 works just like step
2.  Git bisect identifies the following as the responsible commit:

commit 3110159d1fb6b6071803355951d9ae314095df36
Author: Martin Rudalics <rudalics@gmx.at>
Date:   Fri Oct 24 11:58:43 2014 +0200

    Improve mouse dragging of frame edges.
    
    * keyboard.c (make_lispy_position): Return coordinates also when
    on scroll bars, fringes, margins or not in a window.
    * xdisp.c (show_mouse_face): Don't change cursor face during
    mouse tracking.
    * mouse.el (mouse-drag-line): Don't use mouse-pixel-position.
    Calculate increment from last position instead of window edge.
    Add right- and bottom-divider bindings to transient map.



In GNU Emacs 25.0.94.1 (x86_64-suse-linux-gnu, GTK+ Version 3.14.15)
 of 2016-06-03 built on rosalinde
Repository revision: 9c28e70b4b4b1887030e41103b5f6dd8304585e6
Windowing system distributor 'The X.Org Foundation', version 11.0.11601000
System Description:     openSUSE 13.2 (Harlequin) (x86_64)

Configured using:
 'configure --with-xwidgets 'CFLAGS=-Og -g3''

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