Very nice. A few nitpicks :

- You need to wait a bit for a drop target to become active, so when you try to move items quickly it's not registered and snaps back to the start location, is it something that is intended ?

- The animation is smooth with Edge but a bit janky with Firefox, as if it was animated at 10fps or so. It's probably Firefox's fault honestly. Also the border on dragged items is not shown with Firefox.