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

[Andrea Rossetti <andrea.rossetti@gmail.com>]

Andrea Rossetti <andrea.rossetti@gmail.com> writes:
>
>   The new tentative patch is attached here below:
> ....
> +(defsubst ruler-mode-window-col (start-or-end-event)
> ....
> +  (- (car (posn-col-row start-or-end-event))
> +     (if (eq (posn-area start-or-end-event) 'header-line)
> +         (+
> +          (or (car (window-margins)) 0)
> +          (fringe-columns 'left)
> +          (scroll-bar-columns 'left))
> +       0)))

  Some additional word of comment: 

 - I believe the implicit meaning of the above code is 
   "(car (posn-col-row ...)) is fine in text area, but needs
   to be `adjusted' when the drag is performed in header-line
   area";

- whether this adjustment should really be performed by
  posn-col-row or by ruler-mode-window-col is not very clear
  to me; I conjectured that subr.el provides somewhat
  `lower-level' service functions who probably don't want
  to know about window-margins, fringe-columns,
  scroll-bar-columns, but I might be wrong;

- be aware that dragging the column indicators does not work
  if you use non-zero zoom (C-x C-+ or C-x C--); this is an
  independent problem that the patch does not address; the
  issue would need his own bug report.

  Thanks for reading, kindest regards.

Andrea