What could be the reasons for such a desktop bug?

Question

macOS

Good time!
Friends, tell me, is this just a Sierra bug, or what? I am tormented by idle curiosity, I have not come across such things for a long time.
Macbook Pro 2015, a monitor is connected to it with hdmi (actually filmed from it). Macbook is closed, but when "playing a bug", in case of duplication of screens, this happens on the screen of both the monitor and the laptop - I opened it to check.
Can anyone come across? This, of course, does not interfere with the work, but the eye is mazolit, and the reason is simply interesting. It costs Sierra 10.12.1. This only happens on the desktop. You can completely "paint over" this way, and the splash screen will simply become darker as a result of this procedure. After some time, the background is brighter again, and you can draw again.
Video:
https://yadi.sk/i/NyHy3Ox1zxTR8
https://yadi.sk/i/yXYbq4JLzxTwH