Let's say I have a couple of elements on my plot selected already, and I want to select a bunch more. If I hold down the shift key and click on them one at a time, it works fine. But if I hold down the shift key and try to drag a selection box around all of them, it will un-select the items I had selected originally. Since I'm holding the shift key down, I'd expect that it would add to my original selection.
I'm not sure if this counts as a problem or as a new feature, but it's something that I'd like to see changed regardless!