The bug

When combining items in the inventory with shift-double click there need to be free slot(s) in the hotbar according to the number of the resulting stacks.
Visually the items are in fact "parked" for a short time in the hotbar.

How to reproduce

Scenario A

1. Fill your hotbar with any item(s)
2. Have some item spread over the survival inventory, but less than one stack in sum
3. Pick up one of those item
4. Shift double click on another item of the same kind
   → Clicked item will not combine to complete stack

Scenario B

1. Fill your hotbar except one slot with any item(s)
2. Have some item spread over the survival inventory, but more than one stack in sum
3. Pick up one of those item
4. Shift double click on another item of the same kind
   → Clicked item will not combine to complete stack