Changelog

1.0.0

First release. Built against Minecraft 26.2 with Fabric Loader 0.19.3, no dependencies.

• Back slot holding one stack, in the inventory above the shield slot and on the HUD to the right of the hotbar. It appears on the HUD only when it has something in it, matching how the offhand behaves.
• The item is drawn on the player model and is visible to everyone who can see you, not only its owner.
• "Swap Item With Back Item" keybind, default G, in Controls → Inventory.
• Saves with the player, drops on death, honours keepInventory and Curse of Vanishing, and survives respawn on the same terms as the rest of the inventory.
• Usable from the creative inventory's survival tab.

Two details that took some care and are easy to regress, so they're written down:

The HUD slot steps aside for whatever vanilla has already drawn on that side of the hotbar, the offhand ends up there for left-handed players, and the attack indicator does when it's set to hotbar mode, instead of overlapping either.

The item on the back is mounted at two different depths depending on whether the chest slot is filled, because armour is drawn as a separate shell a full pixel outside the body. One fixed depth either floats off a bare back or disappears inside a chestplate.