Show the content of the Scheduling Inbox
In the CalDAV Server-Side Scheduling World Model, invitations between users on the same system are enacted by the server and put in some calendar without interaction from the invitees. If no sufficient privileges are available, the invitations are immediately rejected. Invitations from/to other systems are exchanged via iTIP/iMIP messages.
When a CalDAV server applies implicitly an invitation (puts the iCalendar object in the calendar of the user and tells the organizer about the action, without interaction from the invitee), the recipient of the invitation has a copy of the initial scheduling message (invitation, reply, cancellation) in its Scheduling Inbox. The user can view the iCalendar objects stored in their Scheduling Inbox and delete them.
Evolution shall show the content of the Scheduling Inbox for Collection-CalDAV accounts by adding an additional calendar in the Side bar in the Calendar window. The content of the Scheduling Inbox shall be shown only, when the Scheduling Inbox is selected. Considering that the same event can exist several times in the Scheduling Inbox, but with slightly modified attributes, I think showing the content of the Scheduling Inbox only in List View is the best option. The messages from the Scheduling Inbox are not supposed to be integrated in any calendar together with data from other iCalendar sources. Users shall be able to do only two things with the Scheduling Inbox: view items and delete items.