Odd behaviour when creating private event in shared calendar

[bpcurse]

Events which can only be displayed in the source calendar can be created in a shared version of this calendar with "private" visibility setting without warning and therefore go missing in the shared copy.

Steps to reproduce

1. Create a calendar as user a
2. Share this calendar to user b with write privileges
3. Connect to this calendar as user b via caldav (e.g thunderbird lightning, android davx⁵ and aCalendar or even nextcloud calendar app)
4. Create a new event with visibility "private" as user b
5. Appointment doesn't seem to be created or is visible for a moment and then vanishes
6. Open the calendar of user a
7. Find the new appointment here

Although I understand the logic behind this, I don't think it should be this way. Took me quite a while to find out what's happening and why the appointments created on my mobile and thunderbird were vanishing without notice (with standard visibility setting "private")

Possible solutions
Not sure what a good solution would look like.

1. What about adding an additional sharing option like "can see private events" or "full access"?
2. Do not create event and return a distinct error code? (Maybe not possible at all via CalDAV protocol as I do not know it)
3. Display a hint in a kind of first run wizard for calendar app?

[georgehrke]

Duplicate of #5551