bug#42138: 26.3; Incompatibility between font-lock-add-keywords and enriched.el

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 30 Jun 2020 15:08:22 +0200
> From: Vasilij Schneidermann <mail@vasilij.de>
> 
> After opening enriched.txt and running any kind of code using the
> `font-lock-add-keywords` function (for example whitespace-mode or
> hl-todo-mode), the enriched highlighting is gone.  Is this a bug or
> intentional?  If it's intentional, is there some way for the code using
> `font-lock-add-keywords` to detect this condition, other than checking
> for the presence of enriched-mode?

I think enriched-mode, like any other mode that puts its own faces on
chunks of text by means other than font-lock, is fundamentally
incompatible with font-lock.  It's basically the same problem as if
you tried to use put-text-property in *scratch* to put some face
property on some text in the buffer: the face won't show until you
turn off font-lock.  That's because the first thing font-lock does is
wipe out all the faces in the buffer.

> Alternatively, is there a recommended way to add highlighting of
> keywords that's compatible with enriched-mode?

Any way that uses put-text-property, add-text-properties, etc. without
using font-lock will do.  You can even try that manually via the
facemenu-set-* commands (or via the Edit->Text Properties menu from
the menu bar).