Re: changing the display of formfeed characters

[Javier]

> You could use whitespace-mode itself to display form-feeds. Assuming you
> have whitespace-mode's newline style enabled, try evaling something like
> this before you start whitespace-mode:
> 
> (add-to-list 'whitespace-display-mappings
>              '(newline-mark ?\^L [?= ?= ?=]))

Thanks for the clue.  This works.

But one needs to be aware that whitespace-display-mappings remains
undefined until you invoke whitespace-mode.

(setq whitespace-display-mappings 
      ;;fancy chars from http://ergoemacs.org/emacs/modernization_formfeed.html
      ;; all numbers are unicode     codepoint in decimal. e.g. (insert-char 
182 1)
      '((space-mark 32 [183]  [46]) ; SPACE 32 「 」, 183 MIDDLE DOT 「·」, 46 FULL 
STOP 「.」
        (space-mark 160 [164] [95])
        (newline-mark 10 [182 10]) ; LINE FEED,
        (tab-mark 9 [9655 9] [92 9])) ; tab
        (newline-mark ?\^L  [?= ?= ?= ?= ?= ?= ?=])  ; FORM FEED
)

whitespace-mode now displays everything including ^L formfeed.

whitespace-newline-mode displays only newlines and formfeed.

> If you want the line to extend across the width of your current window,
> you could eval something like this instead:
> 
> (add-to-list 'whitespace-display-mappings
>              `(newline-mark ?\^L ,(make-vector (1- (window-width)) ?=)))
>
> You might also be able to use window-configuration-change-hook or
> similar to adapt the length of the line as you resize the width of your
> window.
> 

This looks like adding more and more entries to the list
whitespace-display-mappings, but only the older one will be taken into
account.