diary/appointment focus-stealing

[Hikaru Ichijyo]

At midnight, if there is a diary file event for the new day, Emacs will
display the event. That much is not unexpected, since the user did say
they wanted notification of events.

However, when Emacs is running on an X windowing system, at least on my
system, it will also suddenly deiconify itself at midnight in response
to any such events for the new day. Not only is this a focus-stealing
behavior (with all the usual unavoidable consequences of
focus-stealing), it is also a very bad thing to happen if another
program like a fullscreen hardware-accelerated 3D game happens to be
running when this happens. The exact specifics of how Emacs suddenly
demanding window focus will be handled varies from game to game, but
it's safe to say that there are few to none cases where it is handled
well. This happens every midnight that there is an event the next day.

It probably shouldn't be necessary for Emacs users to ask themselves
every night around midnight if there's anything Emacs will be
interrupting other programs about coming for the next day, nor should it
be necessary to pre-emptively shut down Emacs around midnight every
night to avoid such a thing happening.

So is there a way this automatic deiconify behavior can be disabled?
I've looked for a variable to set this but wasn't able to find anything.
It's fine that the event/appointment manager brings up a window alert
within Emacs itself; that's what it's supposed to do. It just shouldn't
be reaching outside of Emacs to do things to your X/Wayland windowing
setup unless you decide that's ok, because that's focus-stealing, which
I think is generally evil.

--
He that would make his own liberty secure must guard even his enemy from
oppression; for if he violates this duty, he establishes a precedent
that will reach to himself.
--Thomas Paine