Is the inventory rework just UI, or will it actually add any additional functionality to stacking and unstacking and stuff like that?

There's additional functionality, but obviously the big part of the groundwork was the UI usability. But if you look at the mockups, you also see equipment. So it's not just inventories, actually. So it's not just UI; there's a lot more client architecture also, the way the UI pulls the data, caches the data, and displays it.