OS: Manjaro 16.10.1, KDE 5.8.3, kde frameworks 5.27.0, qt 5.7.0, kernel 4.4.31-2-manjaro, 64 bit, 8gb ram steps to repeat: 1. open System Settings -> virtual desktops -> switching then Animation: Desktop Cube Animation 2. Click on Tools icon, and tick the "Don't animate panels". 3. Switch to a different workspace (either key combination or mouse click on workspaces switcher) 4. Observe a black non-transparent rectangle in top right corner, that covers almost 25% of the screen.
suggests that's actually an invisible panel window?
Created attachment 104720 [details] Black rectangle like this
Happening to me as well on Kubuntu 17.04. Disabling "do not animate panels" seems to fix it. Square is not black for me but it is in the same position.
*** This bug has been marked as a duplicate of bug 374854 ***