In what order are lootboxes opened?

We all know that loot boxes contents are determined when you obtain them. What I want to know is if someone has confirmed in what order they are "served" to you to be opened.

Let me clarify under what circumstances this could be determined. Suppose, you have all Common and Rare items (that means whenever you open a loot box you will get duplicates of such rarity) and that you have saved 5 boxes. The next day Blizzard adds new Non-Event loot (like the Blizzard World loot), but before you touch your 5 loot boxes you win another 5. Now when you open them, what is going to be served to you first? The 5 old crates which will contain mostly duplicates, or the newest 5 which will give you mostly new loot?

Another extra scenario that would reveal this sequence is to have saved event-specific loot boxes from a previous year, along with ones from a brand new event. However this would be a bit harder to try since you would have to have all Common and Rare loot from such event to be able to draw meaningful conclusions.

The oldest lootbox is opened first. I have personally experienced this when opening 3 lootboxes at once as I was getting my last rare items.