Shadow Items Mod

MOD TESTED ONLY ON 1.21.7

I'd appreciate it if you report any bugs on my GitHub

Shadow Items Mod is a mod that adds (or more accurately, restores) shadow items to the game, which became unobtainable after 1.21.1.

Shadow items are items that can exist in two places at once (for example, in two different players' inventories). This isn't a duplication, as there's only one stack (changing one stack immediately triggers an update to the other shadow item).

You can "link" items by trying to move an item to the vortex labeled "Linked." This will visually create two items, but in reality, they'll be shadow items, so everything is fine.

There are several ways to remove a tag from an item (i.e., unlink a specific item): simply take half a stack from the slot, drop it from a dispenser, place the item in a hopper labeled "UnLinked," or drop the item yourself.

The mod has an internal registry that ensures item synchronization across all inventories.
(It scans not only player inventories, but also any entity inventory, including entity blocks. BUT the mod does not scan entity items. If you somehow drop a shadow item, it will not sync with other items.)
This registry records all slots with shadow items and, depending on the internal ID, correctly links the items (technically, it replaces references to Java objects).

Shadow items are extremely stable. They can withstand a server restart or a simple chunk upload. The only weakness of my mod is nested containers. If you take a shulker with a shadow item, the shadow item will be unlinked, but as soon as you place the shulker on the ground, the link will be restored.