1. The feature does not work; right clicking on a shulker box with an item, even though the 'Right Click to Add' prompt is showing, just swaps them.
2. The prompt appears on other shulker boxes when a shulker box is held in the inventory, even though you shouldn't be able to (and can't, due to the previous bug) put one inside another.
   
3. Shulker boxes in the creative inventory allow items to be placed inside them (an exception to bug 1), which seems wrong as they are not actually items yet.
   
   Interestingly, the + and prompt to add item does not render for creative inventory shulker boxes if they have been searched for in the search tab.
4. The cross renders above held items in the inventory - this looks strange and is inconsistent with similar vanilla features such as durability bars.
   

These are issues in ARL, which means you're either not using the latest version, or Vazkii forgot to push it out.