bug#58578: 29.0.50; Font lock randomly breaks in some buffers and gets worse over time

[Stefan Kangas]

Stefan Kangas <stefankangas@gmail.com> writes:

> Here is a screenshot from `notmuch-message-mode', based on
> `message-mode':

For a while, I could reproduce the incorrect fontification in a
`notmuch-message-mode' buffer after replying to an email.

Everything looks fine when I first create the buffer.  But when I move
point to the end of some quoted line, and press RET (`newline'), there
is incorrect fontification on the new line, and I see:

     There are text properties here:
       face                 message-cited-text-1
       fontified            t

(It looks like in the screenshot in the last email.)

If I go to the beginning of another line and press C-o (`open-line'), the
fontification is correct, and there is no face property on the new line:

     There are text properties here:
       fontified            t

Why would `newline' and `open-line' lead to different results?
It seems very strange to me.

[time passes]

What is even stranger is that now, an hour or two later, I can no longer
reproduce this behavior, not in the same session and not even in the same
buffers as before.  The incorrect fontification only still remains on the same
lines as before, however.

I did no particular changes that should affect this.  I was just sending emails
and doing some unrelated ELisp coding.

Should I start looking at my timers here, or something?