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

[Stephen Berman]

On Sun, 15 Jun 2014 21:17:26 +0200 Andrea Rossetti <andrea.rossetti@gmail.com> 
wrote:

>   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

I can reproduce your observation on GNU Emacs 24.3.91.10
(x86_64-suse-linux-gnu, GTK+ Version 3.10.4) of 2014-06-13 on rosalinde.
I can add these observations:

(1) I see # jump whenever the mouse pointer is moved off the ruler while
    holding down mouse-2, regardless of which direction you drag in (but
    conditioned on the following point).

(2) If you don't set the left margin, I see # jump as follows:
    - If you drag from # straight down (due south), # jumps three
      columns to the left. 
    - If you drag from # southeast by one character, # jumps two columns
      to the left.
    - If you drag from # southeast by two characters, # jumps one column
      to the left.
    - If you drag from # southeast by more than two characters, # does
      not jump to the left.
    - If you drag from # southwest, # slides (but doesn't jump) to
      whatever column you stop the drag on.  Moreover, once you drag #
      southwest, you can slide # anywhere while holding mouse-2 with the
      mouse pointer in the text area, just like when it is on the rule.

(3) If you set the left margin, the the number of columns jumped by
    dragging due south equals (margin width + 3), if I'm not mistaken.

Steve Berman