fullscreen controls flickers when screen is under another one

[davidak]

Steps to reproduce:
1. open remmina session in fullscreen mode on a screen that is placed UNDER another one
2. move the cursor to the top of the screen with remmina on, nothing will happen
3. the controls are actually on the screen above the one remmina is on
4. when you hover the control UI, it will move down to the screen where remmina is on but comes back when the cursor is not over it anymore, resulting in constant flickering

Workaround: click on the controls while they are flickering, it will stay on the bottom screen are are usable

See: https://youtu.be/Mg-EB2bloBk?t=44s

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Software: elementary OS 0.4 Loki (based on Ubuntu 16.04 LTS) with HWE Stack
Hardware: MacBook Pro 10.2 from October 2012