Also it's very intermittent, hard to forcefully reproduce. I was doing a test with Xenial on the same laptop earlier today and it never happened.
Also, may be related, may not be: When I first turned off mirror screens so I had the second screen, I ended up with a flickering, messed up background. To get rid of it, I had to full screen something.
Same issue here on a Dell Latitude E5470.
OS: Ubuntu 16.10
Kernel: 4.8.0-22-generic
CPU: Intel(R) Core(TM) i7-6600U CPU @ 2.60GHz
GPU: Intel Corporation HD Graphics 520 (rev 07)
Xorg (package version): 1:7.7+13ubuntu4
Also it's very intermittent, hard to forcefully reproduce. I was doing a test with Xenial on the same laptop earlier today and it never happened.
Also, may be related, may not be: When I first turned off mirror screens so I had the second screen, I ended up with a flickering, messed up background. To get rid of it, I had to full screen something.