In Inventory#tick, the function iterate on all slots in the player inventory to tick them, passing their index as the 3rd parameter and if they are selected as the 4th parameter, but those are not valid for armor or offhand, since the variable 'i' is reset per inventory compartment, resulting in index 0 for offhand and boots, and 1,2,3 for leggings, chest and helmet.

public void tick() {
      for(NonNullList<ItemStack> nonnulllist : this.compartments) {
         for(int i = 0; i < nonnulllist.size(); ++i) {
            if (!nonnulllist.get(i).isEmpty()) {
               nonnulllist.get(i).inventoryTick(this.player.level, this.player, i, this.selected == i);
            }
         }
      }
      armor.forEach(e -> e.onArmorTick(player.level, player));
   }

I think a simple tweak would be enough to fix this since 'compartments' are initialized in order:

public void tick() {
      int index = 0;
      for(NonNullList<ItemStack> nonnulllist : this.compartments) {
         for(int i = 0; i < nonnulllist.size(); ++i) {
            if (!nonnulllist.get(i).isEmpty()) {
               nonnulllist.get(i).inventoryTick(this.player.level, this.player, index, this.selected == index);
               ++index;
            }
         }
      }
      armor.forEach(e -> e.onArmorTick(player.level, player));
   }

 

Why are you reporting a "bug" in Mojang's code?  I am not saying there aren't any bugs in Mojang's code. 🙂

Edited April 11, 20232 yr by warjort

Because this is not a "bug" in itself, but might cause bugs in mods that assume that the index is correct (when using it to replace an item with Inventory#setItem for example). And so Mojang as no reason to "fix" this since it doesn't affect any Vanilla items (only compass and map items use the tick method in vanilla), but Forge might be interested in tweaking this behavior to ensure that mods using the tick method work as intended.

 

You can try to raise it as a real bug report here: https://github.com/MinecraftForge/MinecraftForge

But I guess it would be more likely to get fixed if you submitted a PR?

https://docs.minecraftforge.net/en/latest/forgedev/

Edited April 11, 20232 yr by warjort

Thanks, I will try to do a PR tomorrow then