bug#17788: 24.3; ruler-mode: column indicators "hop" left if you drag them diagonally

[Andrea Rossetti]

  Hello, I found a minor bug in ruler mode, here is
a recipe to reproduce it:

 - start emacs -Q
 - M-x ruler-mode RET
 - click S-<mouse-1> on the column 10 of the ruler, this will set a left
   margin
 - you will see the ruler has two column indicators in red colour:
   "#" for comment column, and "pilcrow" symbol for paragraph fill indicator
 - click and hold down mouse-2 (= center mouse button) on the red "#" indicator
 - drag the indicator horizontally; indicator will follow the mouse as expected
 - continue dragging slightly diagonally (for example in West-South-West
   direction); as soon as you drag the mouse out of the grey ruler area,
   you will see that the indicator "hops" left of about 10 columns
 
  I realize the explanation may be hard to follow because it involves
mouse dragging, so I prepared a small explicative image, it's available
here:

https://raw.githubusercontent.com/thesoftwarebin/the-emacs-software-bin/master/emacs-bugs/ruler-mode-diagonal-drag-issue.png
  
  May someone please confirm if the above recipe seems to be reproducible.

  Thanks in advance for your attention and your patience in reading,
kindest regards. 

  Andrea



In GNU Emacs 24.3.1 (i386-mingw-nt6.1.7600)
 of 2013-03-17 on MARVIN
Windowing system distributor `Microsoft Corp.', version 6.1.7600
Configured using:
 `configure --with-gcc (4.7) --cflags
 -ID:/devel/emacs/libs/libXpm-3.5.8/include
 -ID:/devel/emacs/libs/libXpm-3.5.8/src
 -ID:/devel/emacs/libs/libpng-dev_1.4.3-1/include
 -ID:/devel/emacs/libs/zlib-dev_1.2.5-2/include
 -ID:/devel/emacs/libs/giflib-4.1.4-1/include
 -ID:/devel/emacs/libs/jpeg-6b-4/include
 -ID:/devel/emacs/libs/tiff-3.8.2-1/include
 -ID:/devel/emacs/libs/gnutls-3.0.9/include
 -ID:/devel/emacs/libs/libiconv-1.13.1-1-dev/include
 -ID:/devel/emacs/libs/libxml2-2.7.8/include/libxml2'

Important settings:
  value of $LANG: ITA
  locale-coding-system: cp1252
  default enable-multibyte-characters: t

Major mode: Group

Minor modes in effect:
  diff-auto-refine-mode: t
  gnus-undo-mode: t
  show-paren-mode: t
  server-mode: t
  recentf-mode: t
  cua-mode: t
  tooltip-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-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