I know you can shift drag an item to separate items. Another way to select items is to click on it and press the up/down arrows. The thing is, you had to keep clicking to add or subtract an item. This can become tedious when dealing in items that have large quantities such as screws.

My suggestion is to change this so that you can hold down the arrow and the counter keeps going down or up on its own.

Like I said, a minor suggestion, but I think it will improve the game in a small way.