Apparently MC-224946 got marked as WAI, meaning that items in your cursor won't never drop when you close the inventory UI (unless you're full), items in the crafting grid also won't drop, however this doesn't seem to affect the off-hand slot. If you have an inventory full of items in every slot excluding the off-hand the item will still drop instead of putting it on the off-hand slot.

How to reproduce:

1. Get your inventory full of unstackable items, also drop one on the floor.
2. Open the inventory and get one item in your cursor to get the item on the floor.
3. Close the UI while having the inventory full and the item in your cursor.
4. The item gets dropped instead of putting it on the off-hand slot.