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

[martin rudalics]

>    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 can reproduce the behavior now.  Thanks.

martin