On 2 Mar 2009, at 20:33, Stefan Monnier wrote:

>> I don't know how to fix this given that there seems to be no way to
>> calculate the (average) character width of text with a given face.
>
> That's unfortunate, indeed.
>
>> I haven't encountered such an issue with 22.
>
> The code in Emacs-22, just used a hard-coded 79.

OK, perhaps we could use something like

(if (assq 'default face-remapping-alist) 79  (window-width))

to cover at least the case that I've encountered.

A grep over the source, looking for (window-width), reveals other  
suspicious uses.
I haven't examined it further, but take simple.el, line-move-visual:

       (cond ((eq (nth 1 posn) 'right-fringe) ; overflow-newline-into- 
fringe
	     (setq temporary-goal-column (- (window-width) 1)))

`temporary-goal-column' refers to buffer columns; window-width does not.

Or, abbrev.el:

	(while abbrev-list
	  (if (> (+ first-column 40) (window-width))
	      (progn

first-column seems to refer to the buffer, again.

Or, term.el:
`term-window-width' uses (window-width) and I would assume that this  
(`term-width') is what is used by processes running in the terminal  
(COLUMNS).

Also, there's the longlines-mode case, but I've reported that before  
and we have a suitable replacement thanks to word wrapping.

I think we need something like

"window-buffer-width", giving the average (expected) number of columns  
in a window for text in the default face applicable to the buffer  
shown in a given window.