When exiting your inventory, (E or Esc.) the item you were hold drops out.  I don't know if this is intentional, but when you have an item in a GUI interface (Crafting Table, Furnace, etc.) the items will go back into your inventory without going onto the floor.  However, when you click on an item and leave it drops.  I don't remember this happening in 1.13.

Only tested in Survival Singleplayer.