Cannot edit users when dragging down the list top user

1. Install Nuxeo 7.10
2. Create UserA and UserB
3. Create a Workspace
4. Create a File document
5. Start a serial workflow
6. Head over to the Validation tab
7. In Participants on the Review, add UserA and UserB
8. Observe UserA is at the top of the list and it is possible to enter another username if needed
9. Drag down UserA so that it is below UserB in the list
10. Observe the user widget is reduced and there is no editable space.
11. Drag the UserA back up
12. Observer the editable space is back and usable
13. Drag the UserA down again
14. Observe the user widget is reduced and there is no editable space.
15. Remove UserA and UserB
16. Observe the widget is reduced but there is some editable space. Trying to insert text does not work any longer

After further testing it seems the issue is triggered when dropping one user onto another one. If we drop a user where there is some space left, it works fine.