Interface, Inventory Options (that's at the very bottom) - Always Show Item Stack Splitter.
The tooltip which pops up in the upper right corners says "when trading".
This is not true, as the Splitter appears whenever you rummage in your backpack. E.g. you put all your potions into the Strange Book and you drag the icon for a stack of - let us say 11 - Health potions from one place to another. The game does not simply drop the whole set of 11 bottles there, but asks you how many. This happens both inside the Strange Book and when dragging the stacked items from the Strange Book to the inventory of a different Character.
I am not complaining about HOW it works, but about the misleading description.
