I discovered that freshly created users on my system do not see this problem, and, by bisection, discovered which part of my personal customization causes the effect. Turns out that this was the remnants of my fiddling with Gnome3 shell. Long story short, this command:
dconf reset -f /org/gnome/desktop/background/ brought things to norm. ** Changed in: compiz (Ubuntu) Status: Incomplete => Invalid -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/865676 Title: changing triangular patters on black on background To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/compiz/+bug/865676/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs