Steps to reproduce: - in the left panel, right click and do "New group". Name it "Group A" - again, create another new group and name it "Group B".
Now try to drag and drop Group A into Group B
Expected: should move Group A into Group B. Observed: does nothing. There's no way to move group A into group B (or viceversa).
If group B was an existing group that already contained some repository (or group), then you would be able to drag and drop group A into it (though it's unnecessarily difficult) by moving it in an exact position between two items present in the target group. But it should be possible to do it even when the target group is empty; and whether it is or not, it should be possible to do it by dropping the item onto the group name itself. That's how drag-n-drop works in EVERY other application.
Tested on Ubuntu 14.04.