@Riot do chests have pre-determined contents or no?

I'm wondering because I'm the smart kind of person who doesn't fall for clever antics if I don't have to.

I ask for the following 2 reasons:

Future content may be worth waiting for chest rolls

and I heard something might be done about people who own all champs who keep getting champion shards (which I do)

EDIT: Multiple people saying chest contents are determined at opening, THX!