bug#23571: 25.1.50; Click-mouse-1 = Drag-mouse-1 in Gnus Article buffer

[Stephen Berman]

Clicking mouse-1 in a Gnus Article buffer, then releasing it and moving
the mouse (without holding down a mouse key) highlights a region, just
as when the mouse is dragged with mouse-1 held down.  To reproduce:

0. emacs -Q
1. M-x gnus, type `y' at the prompt, then type `B RET news.gmane.org RET
   1 RET' to open the most recent article in gmane.announce on the
   news.gmane.org server.
2. Click anywhere in the Article buffer (except on the texts following
   the From: and To: headers, which are buttons) with mouse-1, release
   mouse-1, move the mouse.
=> A region is highlighted.

Subsequently clicking and releasing mouse-1 does not deactivate the
mark (but clicking with mouse-2 or mouse-3 does).

I have not observed this behavior anywhere besides Gnus Article buffers.

This happens in master but not in emacs-25.  It happens at least since
commit 62d7acae7405732268713006d839a5c3507b9482, which was my first
build from master after a long pause, so I don't know when this behavior
first appeared (I didn't save any earlier builds from master, which were
from many months before, but I'm sure they didn't show this behavior).

In GNU Emacs 25.1.50.4 (x86_64-suse-linux-gnu, GTK+ Version 3.14.15)
 of 2016-05-17 built on rosalinde
Repository revision: 631ca55c6decccca2dc0961dc28962819eacc35b
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

Important settings:
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix