Starting to drag item stacks over other compatible stacks makes the latter invisible until appearance change (stack size increases)

To reproduce:

1. Get a 64 of an item
2. Split it into 2 stacks of 32 and put both stacks in different inventory slots
3. Left click one of the stacks
4. Move your mouse over the other stack
5. Left click your mouse and keep it clicked
6. Move your mouse a tiny bit without leaving the slot
7. Observe how the stack that was in this slot is now invisible
8. Move your mouse to another slot with nothing or the same type of item in it
9. The stack is now visible again

Also see attached screenshots.