JabRef 3.7dev--snapshot--2016-09-29--latex-free-fields--93e665e
windows 10 10.0 amd64
Java 1.8.0_101

1. Create two new groups
2. Try to drag'n'drop one of the groups onto the other group to create a group-subgroup structure
3. Nothing happens, i. e. no subgroup is created
4. The only way to create subgroups is to use the "Move" and "Right"/"Left" command

This issue might be a duplicate of this one here #2098, but I wasn't sure, so I thought to better report it separately.