bug#29538: 26.0.90; Gnus, group buffer: Mouse click on empty space selects group

[Benjamin Riefenstahl]

The behaviour of mouse clicks has changed in the Gnus group buffer from
Emacs 25 to Emacs 26.

In Emacs 25, when I clicked on empty space to the right of lines or
after the last group, this did nothing.  This was usefull to just
activate the Emacs frame.

In Emacs 26, clicking after lines opens the group on the line where I
clicked or it opens the last group, if I click in the space after the
last group.  As I use the mouse mostly to activate Emacs and once Emacs
is active I use the keyboard for most of my interaction, this is a
rather annoying change for me.

I would guess that for some reason, in Emacs 25 the lineend was inactive
and in Emacs 26 it is now activated for mouse clicks.  I currently do
not even understand how activation for mouse clicks work in this buffer,
so I can't provide a fix myself.

---

In GNU Emacs 26.0.90 (build 1, i686-pc-linux-gnu, GTK+ Version 2.24.25)
 of 2017-12-02 built on justinian
Repository revision: 8227087194e0817b984ce3b15099f5eae4dc011c
Windowing system distributor 'The X.Org Foundation', version 11.0.11604000
System Description:     Debian GNU/Linux 8.9 (jessie)

Recent messages:
Reading active file via nnfolder...
Reading incoming mail from file... [2 times]
Reading incoming mail from pop...
nnfolder: Reading incoming mail (no new mail)...done
Reading active file via nnfolder...done
Reading active file via nndraft...done
Checking new news...done
When done with this frame, type C-x 5 0
Quit [2 times]
Type C-x 1 to delete the help window, C-M-v to scroll help.

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND GPM GSETTINGS NOTIFY LIBSELINUX GNUTLS
LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB TOOLKIT_SCROLL_BARS GTK2 X11

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Help

Minor modes in effect:
  desktop-save-mode: t
  delete-selection-mode: t
  display-time-mode: t
  diff-auto-refine-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  buffer-read-only: t
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
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gnus-score score-mode nnvirtual gnus-msg nntp gnus-cache pop3 nnrss
gnus-art mm-uu mml2015 mm-view mml-smime smime dig gnus-sum gnus-group
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cus-edit cus-start cus-load eww mm-url gnus nnheader wid-edit url-queue
url url-proxy url-privacy url-expand url-methods url-history url-cookie
url-domsuf url-util mailcap shr svg xml dom edmacro kmacro conf-mode
imenu elec-pair jka-compr desktop frameset highline benny-calendar-cfg
ange-ftp benny-unicode generic-x autoinsert cc-cmds cc-engine cc-vars
cc-defs ps-print ps-print-loaddefs ps-def lpr advice benny-url cmuscheme
comint ansi-color ring scheme delsel disp-table time server protbuf
cal-china lunar solar cal-dst cal-bahai cal-islam cal-hebrew holidays
hol-loaddefs vc-git diff-mode easy-mmode vc-fossil vc vc-dispatcher
diary-lib diary-loaddefs cal-menu calendar cal-loaddefs benny-file-cache
message-x message rmc puny dired dired-loaddefs format-spec mml mml-sec
epa derived epg gnus-util rmail rmail-loaddefs time-date mm-decode
mm-bodies mm-encode mail-parse rfc2231 gmm-utils mailheader bbdb-snarf
mail-extr rfc822 bbdb-com mailabbrev bbdb-autoloads bbdb cl timezone
sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
.loaddefs browse-url autoload radix-tree lisp-mnt finder-inf gh-common
gh-profile rx s marshal eieio-compat ht json map dash info package
easymenu epg-config url-handlers url-parse auth-source cl-seq eieio
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select scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame cl-generic cham georgian utf-8-lang misc-lang
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chinese composite charscript charprop case-table epa-hook jka-cmpr-hook
help simple abbrev obarray minibuffer cl-preloaded nadvice loaddefs
button faces cus-face macroexp files text-properties overlay sha1 md5
base64 format env code-pages mule custom widget hashtable-print-readable
backquote inotify dynamic-setting system-font-setting
font-render-setting move-toolbar gtk x-toolkit x multi-tty
make-network-process emacs)

Memory information:
((conses 8 430699 23066)
 (symbols 24 152994 2)
 (miscs 20 106 348)
 (strings 16 204009 6852)
 (string-bytes 1 5349834)
 (vectors 12 38477)
 (vector-slots 4 851193 36246)
 (floats 8 1019 104)
 (intervals 28 629 0)
 (buffers 536 28)
 (heap 1024 57025 1736))