multi-monitor - drag and drop triggers bug #748539 (unity on top of full screen apps)

[groucho]

Setup:
2 monitors
ubuntu 14.04 and ubuntu 16.04, fresh install and default settings

Steps:
- start a new session
- open nautilus on monitor 1
- open a full screen app on monitor 2
- do a drag and drop action in nautilus (moving a file or a folder)
- select an app on monitor 1

Expected result:
- the full screen app stays on top

Actual result:
- the unity interface appears on top of the full screen app ( bug #748539 ). This behavior will remain until the end of the session.

Nautilus is just an example, drag and drop actions in other apps trigger the bug too.