[0.17.1] Items from the cursor can't be dropped into the slot reserved for the cursor

[mrvn]

Say I have 11 iron plates in my inventory and I want to place 10 into an assembler. I open the inventory and pick up the iron plates. Now the inventory has one slot reserved for the cursor and the cursor holds 11 iron plates, one too many. So I want to put one back into the inventory. But trying to put the plate into the reserved slot gives an error that the slot is reserved for the cursor. If the inventory is otherwise full this makes it impossible to adjust the amount held in the hand.

[kovarex]

The reserved slot must be fully empty, you can put it in other empty slot if there is any.