Smart Stack Off-Hand Extension is…well, as the name suggest, an extension from the add-on Smart Stack.

This add-on refills your last item or block from your Off-Hand. This add-on is designed to be compatible with Smart Stack and Double Hands for full experience. (Or…you can just use this alone and use the Vanilla supported items on the Off-Hand)

Please READ, these are important notes you need to know

• The add-on doesn't duplicate, delete items from your inventory, survival friendly!

• The add-on won't refill non-stackable items to avoid overwriting any enchanted items like shields or similar, leaving with one only exception, it will refill Totems of Undying.

• Move the Item/Block from your Off-Hand to your inventory: Due to engine limitations, if you want to move the item manually (leaving the Off-Hand empty) when opening the inventory, as soon as you move the item/block from the Off-Hand to any of the inventory slots, it will immediately auto-refill your Off-Hand with that same item. To successfully move the item from your Off-Hand to any of your inventory slots, you will have to move it twice to overrun the add-on time it has to attempt refilling the item, this means when you move the item from your Off-Hand to your Inventory it will reappear (refill) in your Off-Hand again and that's when you move it again fast.(I have tested it with for Mobile, Controller and Mouse/Keyboard, it's easier on Controller and Mouse/Keyboard since you have a button/key to quickly move items. TIP: for mobile users: if you have a resource pack like Deésse UI, you can use the button quick move to move the Off-Hand item to your Inventory, the resources pack is not necessary but it makes it more comfortable to move the item from the Off-Hand to the inventory)

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• Credits:I used as reference the add-ons Double Hands and  Smart Stack Stack code to make this add-on. I did not copy and pasted the original codes from the add-ons to this one, I made the script from scratch but still used key references:

• Smart Stack: Detect item in MainHand then find the same item ID to refill.
  Double Hands: To move items from MainHand to OffHand it uses the command /replaceitem (@minecraft/server) which was essential to make this add-on exist in the first place.
  
  DISCLAIMER: This add-on was made from scratch and it doesn't reuse code from the other add-ons, I did not copy & paste any of the code lines.