I have a shared calendar with my wife, using CalDav.
When a Reminder window opens (on both our computers) and one of the Reminders is Closed, on the other computer the window is still open.
When then that window is closed (even with the [X]) it warns that something has changed, and if the closing of the window should be transmitted to the CalDav system, or if you want to 'reload' the data.
This does not make sense to me. Once the Reminder is Closed, it should also be closed on all other computers, and it should automatic close all the open windows (of the same Reminder) and this will prevent the problem of possible duplicate and conflicting commands to the sharing system.