Get a stack (preferably) of any material and place this material in any slot you wish. Next, on an empty slot, switch to the slot you put your materials in while holding your drop key on your keyboard without letting go. You'll notice that the item drop animation plays an the amount of the aforementioned material has decreased.
MC-166415
Visual issue with drop key (whilst dropping)
Open
1
(Unassigned)
Confirmed
(Unassigned)
1.15 Pre-Release 2, 1.15 Pre-release 4, 1.15 Pre-release 6, 20w07a, 1.16.1, ..., 1.16.2 Pre-release 3, 1.16.2 Release Candidate 2, 1.16.2, 1.16.3 Release Candidate 1, 1.16.3
Retrieved
Able to reproduce in 1.15 pre-6 with very minimal consistency. 90% of the time it works as intended, the other 10% this bug happens.