Two things bugged me about the Inventory UI, especially when you want to swap two items in your inventory, but have to shuffle things around to do it...
Scenario: You want to swap a Medkit with a pistol. The Medkit is in your backpack, the pistol is on your belt, but there are too many things in both the backpack and belt to simply swap them.
So, you have to drop the pistol on the ground, move the Medkit from backpack to belt, move the pistol from the ground to the backpack.......and you're charged the AP for each of those moves.
The more practical way of doing it from the user's point of view is to click on either the pistol or Medkit, move it over top the other, and drop it. When you do that, the other item is now selected and you can move it to the other location, without the need to drop anything.
The second scenario is when you're just generally moving stuff around and you make a mistake. You switch things back to the way they were...but you're still charged all the AP costs. Seems to me the total AP cost for interacting with your Inventory should be determined not by each individual action from A to N, but rather the AP cost of what's changed.
Scenario: I move a pistol from my belt to my backpack, then back to my belt, then drop it on the ground, then equip it as a weapon...then put it back on my belt. In the end nothing's changed. You were just trying different things out, but it's counted against you. Just seems a bit mechanistic.