Can't test for items in the 'player.cursor' slot in Creative mode

As we already know, we can set items in the 'player.cursor' slot with the command :

item fill entity @s player.cursor with dirt

Open your inventory and you’ll have a dirt block on your cursor. But it’s impossible for us to test for any item in this slot…

If we try to set any other slot from this one, nothing happens : (put in a repeating command block)

execute as @p run item replace entity @s armor.head from entity @s player.cursor

And even predicates aren’t able to test for items here : (example for a slot_source)

{
  "type": "minecraft:filtered",
  "item_filter": {
    "items": "minecraft:knowledge_book",
    "count": 1
  },
  "slot_source": [
    {
      "type": "minecraft:slot_range",
      "source": "this",
      "slots": "hotbar.*"
    },
    {
      "type": "minecraft:slot_range",
      "source": "this",
      "slots": "armor.*"
    },
    {
      "type": "minecraft:slot_range",
      "source": "this",
      "slots": "inventory.*"
    },
    {
      "type": "minecraft:slot_range",
      "source": "this",
      "slots": "weapon.offhand"
    },
    {
      "type": "minecraft:slot_range",
      "source": "this",
      "slots": "player.cursor"
    }
  ]
}

Using this slot source, if we try to replace any knowledge book in the player's whole inventory, the book will be replaced in every slot it occupies, except for the 'player.cursor'.

To test this, place the following command in an Impulse Command Block with a pressure plate on top. Then, fall from a great height onto the pressure plate to give yourself time to open your inventory and pick up the item with your cursor.

execute as @p run item fill entity @s example:knowledge_book_in_player_inventory with dirt

To give yourself a knowledge book, run this command in the chat:

give @s minecraft:knowledge_book

You can test this command and slot_source by installing the provided datapack.

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