Subject: Re: bug#635: Adding font-lock keywords results in no font-lock at all

From: martin rudalics <[email-masked]>
Date: Tue, 05 Aug 2008 15:12:46 +0200

> For many years I had the following statement in my .emacs file:
 >
 > (defun show-tabs () "Show tabs with a slightly changed background"
 >   (font-lock-add-keywords nil '(("\t"  (0 'tab-face t))) t))
 > (add-hook 'font-lock-mode-hook 'show-tabs)
 >
 > This has worked without any problems up to 22.1.
 > In version 22.2 and 23 I do have problems with syntax highlighting due
 > to these few lines above in some modes, e.g. when I try M-x list-faces-display
 > all colors are gone.

The behavior you observe seems due to the change

2007-06-11  Stefan Monnier  <[email-masked]>

	* font-lock.el (font-lock-add-keywords): In case font-lock was only
	half-activated, forcefully activate it completely.

which added the following lines to `font-lock-add-keywords'

          (when (and font-lock-mode
                     (not (or font-lock-keywords font-lock-defaults)))
            ;; The major mode has not set any keywords, so when we enabled
            ;; font-lock-mode it only enabled the font-core.el part, not the
            ;; font-lock-mode-internal.  Try again.
            (font-lock-mode -1)
            (set (make-local-variable 'font-lock-defaults) '(nil t))
            (font-lock-mode 1))

What happens appears to be that in any mode that does not specify
`font-lock-keywords' (like `help-mode' or `view-mode' in your case),
(font-lock-mode -1) unfontifies the entire buffer thus removing any
faces assigned earlier by, for example, `list-faces-display'.  Since
these faces are not reassigned by font-lock you won't get them back
either :-(

AFAICT, this change also broke `hi-lock-mode' in such buffers.

Get your original behavior back by writing something like

(defun show-tabs () "Show tabs with a slightly changed background"
   (unless (or font-lock-keywords font-lock-defaults)
     (set (make-local-variable 'font-lock-defaults) '(nil t)))
   (font-lock-add-keywords nil '(("\t"  (0 'tab-face t))) t))
(add-hook 'font-lock-mode-hook 'show-tabs)

or by removing the offending lines from font-lock.el ;-(

martin