If you single click (not hold) items in your inventory to move them, and very quickly move to another cell or outside your inventory, the game will drop the item you clicked into whatever cells your cursor is now covering without clicking a second time. This only happens when you move the cursor very quickly.
It seems more likely to happen at higher FPS. It can be done at 60fps, 15fps seems impossible.
I think the same thing was reported years ago, here: Clicking an item in your inventory and then quickly moving your mouse causes you to "drop" the item
I had my friend test and she said it was also reproduceable.
I was able to do this with two different mouses, and tested to ensure they were only clicking a single time.
Unfortunately this is how I always click in these games, so for the time being I’ll get used to holding the mouse button down, but I’d really appreciate this being fixed as my muscle memory will keep giving me this issue.
Thank you!