In survival mode, when the inventory is full, the cursor can act like an extra temporary inventory slot while the inventory is closed.

 Steps to reproduce

1. Fill your inventory up. At least one of these items should not stack with any other items in other inventory slots, e.g. a sword.
2. Have a dropped item on the ground, e.g. some cake, that you cannot pickup due to your full inventory.
3. Open your inventory.
4. In quick succession, click the sword, and immediately close the inventory. When you succeed, notice that the sword is not thrown on the ground, while the cake is picked up.
5. To ensure that this is not just a visual bug, move a distance away and open your inventory again.
   → The sword is still on your cursor

The sword functions normally after step 5, i.e. it can be automatically/manually dropped from the cursor, and can be placed back in the inventory.