Support for virtual folders without crashing, more precisely, support more than one item with the same id.
I started by changing the hash keys from Item::Id to Item::Id,Collection::Id but soon realized the mistake it was, it touched lots of code, and we're a bit near the 4.6 release to introduce instability.

So basically I added a new structure where we keep track of these dups. Working fine in monthview. Agenda needs a tweak, it's missing add/remove notifications.