When I right-click on an item stack, among other things the command “Stack all” is offered.
But if I want to split a stack, I have to drag it while holding down the Shift-key.
If you can use only one hand, this is very difficult.
Could not also be offered the command “Unstack” in the menu by right-clicking on an item stack?
You could then split the stack and drag it to where you want afterwards.