bug#18212: 24.3.92; Left-click on a button is treated as mouse-2

[Matthew Woodcraft <matthew@woodcraft.me.uk>]

If click with my left mouse button (normally mouse-1) on "button" text,
Emacs appears to think I clicked mouse-2 (normally the middle button).


Recipe to reproduce:

emacs -Q --exec "(global-set-key (kbd \"<down-mouse-2>\") 'clipboard-yank)" --exec "(info)"

Left-click on one of the hyperlinks.

Emacs says
  Buffer is read-only: #<buffer *info*>
rather than following the hyperlink.


Using describe-key with a left click suggests that Emacs thinks that,
when over a 'button', the left click is mouse-2:
«
<down-mouse-2> at that spot runs the command clipboard-yank, which is
an interactive compiled Lisp function in `menu-bar.el'.
»
If I click somewhere that isn't a 'button', Emacs thinks it's mouse-1.


The problem didn't happen with the released Emacs 24.3.

This Emacs was built from the emacs-24 branch as of 2014-08-03.

In GNU Emacs 24.3.92.1 (i486-pc-linux-gnu, GTK+ Version 3.4.2)
 of 2014-08-03 on golux, modified by Debian
 (emacs-snapshot package, version 2:20140803-mjw1)
Windowing system distributor `The X.Org Foundation', version 11.0.11204000
System Description:	Debian GNU/Linux 7.6 (wheezy)

Configured using:
 `configure --build i486-linux-gnu --host i486-linux-gnu --prefix=/usr
 --sharedstatedir=/var/lib --libexecdir=/usr/lib --localstatedir=/var
 --infodir=/usr/share/info/emacs-snapshot --mandir=/usr/share/man
 --with-pop=yes
 --enable-locallisppath=/etc/emacs-snapshot:/etc/emacs:/usr/local/share/emacs/24.3.92/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/24.3.92/site-lisp:/usr/share/emacs/site-lisp
 --enable-silent-rules --enable-link-time-optimization
 --with-crt-dir=/usr/lib/i386-linux-gnu/ --with-x=yes
 --with-x-toolkit=gtk3 --with-toolkit-scroll-bars --with-imagemagick=yes
 'CFLAGS=-g -O2 -fstack-protector --param=ssp-buffer-size=4 -Wformat
 -Werror=format-security' CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-z,relro'

Important settings:
  value of $LC_CTYPE: en_GB.UTF-8
  locale-coding-system: utf-8-unix