bug#28599: 26.0.50; font-lock-ensure changes the face of text buttons

[Lars Ingebrigtsen <larsi@gnus.org>]

Alexander Miller <alexanderm@web.de> writes:

> Steps to reproduce:
>
> * emacs -q
> * switch to a text-mode buffer
> * Run (insert-text-button "AAAAAAAAAAA" 'face 'font-lock-constant-face)
> * Run (font-lock-ensure)
>
> The button's face will now have changed from font-lock-constant to button.
> This only happens once, a second call to font-lock-ensure makes no such
> changes.
> Overlay buttons don't seem to be affected.
>
> I also tested this on a self-compiled emacs 25.2.1, with the same result.

I'm unable to reproduce this exactly -- when I try this in Emacs 27, the
face property is just cleared, and no new face is applied.  And it
happens on all calls to font-lock-ensure.

This happens because that function calls,
font-lock-default-fontify-region, which calls
font-lock-default-unfontify-region is called, and it just does

(defun font-lock-default-unfontify-region (beg end)
  "Unfontify the text between BEG and END.
This function is the default `font-lock-unfontify-region-function'."
  (remove-list-of-text-properties
   beg end (append
	    font-lock-extra-managed-props
	    (if font-lock-syntactic-keywords
		'(syntax-table face font-lock-multiline)
	      '(face font-lock-multiline)))))

So if you want to have both your own faces and use font-lock, too, you
have to use font-lock-face instead of face, according to "Precalculated
Fontification".

So there doesn't seem to be a bug here.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no