Eli Zaretskii <eliz@gnu.org> writes:

How did you see that, and where?

Mostly in third-party major-mode. But also in built-in ones, for example css-mode that have font-lock-keywords-case-fold-search set to 't.

I haven't seen any minor mode that uses this variable. I guess the reason is to avoid conflicts with the major mode.

We need to understand the issue better to reply to this question. So please provide more details.

My idea is to add a kind of note to font-lock-keywords-case-fold-search documentation in lispref tex file to avoid any confusion that this can be used in minor modes.

Something like this:

diff --git a/doc/lispref/modes.texi b/doc/lispref/modes.texi
index 768d78229aa..02e7a6f553f 100644
--- a/doc/lispref/modes.texi
+++ b/doc/lispref/modes.texi
@@ -3573,6 +3573,9 @@ Search-based Fontification
 @defvar font-lock-keywords-case-fold-search
 Non-@code{nil} means that regular expression matching for the sake of
 @code{font-lock-keywords} should be case-insensitive.
+
+@strong{Warning:} This variable is intended to be used by major modes
+and it's not recommended change it by minor modes
 @end defvar

 @node Customizing Keywords