RE: changing the display of formfeed characters

[Drew Adams]

> AFAICT, those modes break whitespace-mode because they define their
> own display table without copying the display table that was in effect
> before the mode was switched on.  

FWIW, `pp-c-l.el' creates a display table only when
`(window-display-table WINDOW)' is nil. Otherwise, it just sets the
entry for ^L in that existing display table:

(walk-windows
     (lambda (window)
       (let ((display-table  (or (window-display-table window)
                                 (make-display-table))))
         (aset display-table ?\014 (and pretty-control-l-mode
                                        (pp^L-^L-display-table-entry window)))
         (set-window-display-table window display-table)))
     'no-minibuf
     'visible)