How to get rid of this UI "feature"/bug(without rebooting)?

Please look at this strange border - I have Firefox opened, as usual, but after pressing some keys a strange border appeared(can be seen between the dash icon and the google logo, overlapping the Firefox window) :

highlighted ->

Note that this border overlaps everything, not only the firefox window, bot also the background wallpaper etc. and can become quite annoying.

This border stretches over the entire screen, but I didn't want to bother you with a 1900x1200 screenshot...

What can I do to "fix" this, ie remove that ugly thing(ideally without rebooting or logging off) ?

Plus : Is this a bug or a feature ?

My version is 16.04.

Thanks in advance for your time and possible help.

EDIT : Rebooting fixes this. Question still persists.