> The variable doesn't work as is (because of the problem of mixed
> frames), and `auto-detect' doesn't have much meaning, which is why this
> should never have been a variable in the first place.
>
> Instead of trying to fix that mess, I thought it would be easier to
> introduce a new function that does the right thing automatically, and
> without a gazillion optional parameters, and then make the old function
> obsolete.

I have no opinion about a new function.  Like everyone else, I'm using
truncate-string-to-width, and happy with it, except of one complaint:
on every use I need to wrap it with such code:

  (let ((ellipsis (cond
                   (truncate-string-ellipsis)
                   ((char-displayable-p ?…) "…")
                   ("..."))))
    (truncate-string-to-width string max nil nil ellipsis))

Preferably, this should be fixed with this patch: