Sometimes if you move the mouse too fast and click inside the hotbar slot and then move your mouse outside the window (normally I do that because of the momentum of the mouse 😛) it will register the user as clicking outside the inventory window rather than in the hotbar slot.
This isn't due to a large tick time and the game thinking the mouse was outside the window at the time because I am running at ~200fps so there is no way I could have moved the mouse that far in 1/200th of a second. Besides the time between clicking and it dropping is clearly visible (around 1/10 of a second I'd guess).
It looks like this may be due to the new dragging items feature waiting to see if it is a drag or click. If so I'd recommend saving the previous location of the mouse at the start of a drag and using that position data if it was actually just a "drop".
Sorry if this is a duplicate but I wasn't sure how to word the search and couldn't find anything.
Duplicate of MC-8625.