Description of Bug: Liquids can be poured out from a container into that same container in the inventory menu. I tested with water on flasks and bottles. I think it should work with all liquid containers with a capacity more than one cup?
Reproduction Steps:

1.  Drink out of a full container / Find a container that contains water, but isn't full
2.  Drag out of the inventory, while holding down click, drop it back down where the item is in the inventory
3.  Container is refilled.

Screenshots: