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

[Stephen Berman]

On Mon, 16 Jun 2014 01:06:55 +0200 Andrea Rossetti <andrea.rossetti@gmail.com> 
wrote:

>   Thanks Stephen for having tested at your side.
>
>> (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
>>     ...
>
>   Yes, (1) is the same for me.
>
>> (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.
>>
>
>   I can't replicate any of the cases in (2); I always get 1 column hop.
>
>> (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.
>
>   In my setup I see the indicator jumps left for (margin width + 1)
> columns. I'm afraid some other window parameters may be involved,
> for example:
>
>   - if you have a leftside fringe (I have it)

Me too (as well as right fringe).

>   - if you have a leftside scrollbar (I only have it rightside)

I do indeed that the scroll bar on the left (it's also a non-toolkit
scroll bar, FWIW).

>   - (maybe?) if your window manager sets thick window borders
>     (I have just 3-4 pixel borders)

Do you mean X-window borders, i.e., Emacs frame borders?  If so, I have
one pixel wide borders.

>   I wrote a (quite rough) patch that seems to resolve all "hop"
> problems on my installation. May I please ask if anyone has time
> and patience to try the patch and report if it provides some benefit.

With your patch, I still get a jump of # when I drag the mouse pointer
down out of the ruler, but now only by two columns to the left,
regardless of whether I set the left margin or not.

> -(defsubst ruler-mode-window-col (n)
> +(defsubst ruler-mode-window-col (start-or-end-event)
>    "Return a column number relative to the selected window.
> -N is a column number relative to selected frame."
> -  (- n
> -     (car (window-edges))
> -     (or (car (window-margins)) 0)
> -     (fringe-columns 'left)
> +START-OR-END-EVENT is the start point or end point of a mouse
> +click or mouse drag event.
> +                                                                  
> +See also: `event-start', `event-end', `posn-col-row', `posn-area'.
> +"                                                                 
> +  (- (car (posn-col-row start-or-end-event))                      
> +     (if (eq (posn-area start) 'header-line)
                           ^^^^^
I guess this should be `start-or-end-event' (otherwise the byte compiler
warns about a free variable).

> +         (+ (or (car (window-margins)) 0) (fringe-columns 'left)) 
> +       0)                                                         

Steve Berman