It seems splitting item quantities with <shift + left mouse button> only works properly when you press the shift key first, and then the left mouse button. Otherwise, you move the entire item cargo and the splitting cargo dialog doesn't appear. This behaviour isn't very user friendly, but I'm not sure if is a bug or it is intended...

If it's intended, it's very sloppy. Also, the regular drag and drop can be very inexact, it's frustrating.