bug#39597: 27.0.60: M-x occur adds fontification to fundamental-mode

[Juri Linkov]

> Maybe something like the patch below?
> I'm not sufficiently familiar with the design of `font-lock-specified-p`
> to be completely sure where the check should be placed; this is my best guess.

> diff --git a/lisp/font-lock.el b/lisp/font-lock.el
> index 438511b647..bf045d9d7e 100644
> --- a/lisp/font-lock.el
> +++ b/lisp/font-lock.el
> @@ -1139,9 +1139,10 @@ font-lock-ensure
>    "Make sure the region BEG...END has been fontified.
>  If the region is not specified, it defaults to the entire accessible
>  portion of the buffer."
> -  (font-lock-set-defaults)
> -  (funcall font-lock-ensure-function
> -           (or beg (point-min)) (or end (point-max))))
> +  (when (font-lock-specified-p t)
> +    (font-lock-set-defaults)
> +    (funcall font-lock-ensure-function
> +             (or beg (point-min)) (or end (point-max)))))
>  
>  (defun font-lock-default-fontify-buffer ()
>    "Fontify the whole buffer using `font-lock-fontify-region-function'."

I confirm this fixes the problem reported by Kévin,
so maybe this should be installed to emacs-27.