I have multiple calendars in Thunderbird. When I get an invitation for an event and press decline, Thunderbird asks me to select a calendar (which is a bit odd, but o.k.). After selecting a calendar, the declined event is, in fact added to my calendar, and displayed as semi-transparent. Is there a possibility to decline events without adding them to one of my calendars? The point of declining an invitation is not to add it to any calendar.