This hack has been added ages ago to "speedup calendar display".

That was added to load and cache user groups in the session during the login so when the user visit the calendar the group list is available in global $USER object and can be easily retrieved.

The problem is that it doesn't pick up group and override changes, keeping the same groups from the time of login and because of that, newer creater overrides events won't be displayed in the calendar.

This ticket is about properly removing that hack, we should use proper Group API function groups_get_user_groups() to get the list of groups the user belongs to.

We should also add a mention to upgrade.txt