A strange black square appears at the top-left side of the screen. I log in, it does not appear until I open Whisker menu, but whenever I open it, it simply appears and stays at the top of all application windows.
It is not a functional problem but it is getting annoying. This is the screenshot while Whisker menu is open:
It might sometimes imitate the wallpaper, ss:
Xubuntu 12.04.5 user here, I launched xwininfo command from a shell, and point to the black rectangle, it says:
xwininfo: Window id: 0x240007d (has no name)
launching command xkill -id 0x240007d did close the black rectange.
The window id is not the same every time: this time wininfo shows an id of 0x360007b.
Moreover, I played an openGL game and the rectangle is no more black, but shows a bit of game graphics.
Try in a terminal
xfconf-query -c xfwm4 -p /general/use_compositing -t bool -s false
xfconf-query -c xfwm4 -p /general/use_compositing -t bool -s true
The square disappears, but you need to do after each reboot of the computer.
I found this solution on another forum. I don't understand why that works.
There's another permanent solution to this issue.
If you install another Ubuntu with desktop environment (like Kubuntu) and then install Xubuntu and completely uninstall the old environment, the black square will not appear.