I'm trying to make a backpack item that inventory size changes depending on a certain enchantment level. For this reason, I would like to resize the inventory of the item stack when enchantments are granted or cleared. Is there an event that is commonly called for them? Is there no choice but to do enchantments by gold floor, enchantment table, command, etc. at each event? Or should I always check the item's enchantment level and inventory size on every tick?

I want to drop the overflowing contents on the spot when the enchantment level is lowered by a whetstone or other means. Is there a way to get the owner's player with getCapability ()?

Yes. The itemstack may be in the player's inventory or dropped as an item entity. At least, what's the best way to drop an overflowing item when the enchantment is stripped with a grindstone?

@Override
public void inventoryTick(ItemStack stack, World worldIn, Entity entityIn, int itemSlot, boolean isSelected) {
	if (!worldIn.isRemote()) {
		if (stack.getItem() != UMUBackpackItems.BACKPACK.get()) return;
		if (!(entityIn instanceof PlayerEntity)) return;

		IItemHandler inv = stack.getCapability(CapabilityItemHandler.ITEM_HANDLER_CAPABILITY).orElseThrow(RuntimeException::new);
		int size = BackpackItem.getSizeFromLevel(EnchantmentHelper.getEnchantmentLevel(Enchantments.BACKPACK_CAPACITY, stack));

		if (inv.getSlots() == size) return;

		entityIn.sendMessage(new StringTextComponent("slot:" + inv.getSlots()), null);
		entityIn.sendMessage(new StringTextComponent("size:" + size), null);

		NonNullList<ItemStack> oldInv = NonNullList.withSize(inv.getSlots(), ItemStack.EMPTY);
		for (int i = 0; i < inv.getSlots(); i++) {
			oldInv.set(i, inv.getStackInSlot(i));
		}
		((ItemStackHandler) inv).setSize(size);

		for (int i = 0; i < inv.getSlots(); i++) {
			if (i < oldInv.size()) {
				((ItemStackHandler) inv).setStackInSlot(i, oldInv.get(i));
				oldInv.set(i, ItemStack.EMPTY);
			}
		}

		oldInv.forEach((s) -> ((PlayerEntity) entityIn).dropItem(s, true));
	}

	super.inventoryTick(stack, worldIn, entityIn, itemSlot, isSelected);
}

public static int getSizeFromLevel(int level) {
	return 9 * (1 + level);
}

So I want to compare the enchantment level with the current inventory size in Item#inventoryTick and resize the inventory correctly when they don't match. However, whenever I open my backpack inventory, I only see 9 slots. (chat with slot: 9, size: the correct size) I thought that the cause is that the NBT is not updated, but I do not understand the hierarchical structure under ForgeCaps of the NBT in the item stack.

Do you think this is due to NBT? Also, if so, please tell me the structure of capabilities up to NBT.