On Sat, 03 Aug 2019 09:50:56 +0200
tastytea@tastytea.de wrote:

> Maybe whitespace-mode could watch fill-column for changes, at least
> until the file-local variables are loaded?
> There is also the possibility that fill-column is changed afterwards by
> the user, but I guess that's not too common…

Ah, the new variable watcher thing. I suppose that would be a
possibility, though so far its use in Emacs seems to be quite limited.

Less avant-garde perhaps would be to do what's being done for some of
the other regexps already: turn it into a function: