Screensaver issue ? OR user issue?

Hello everybody and happy new year !

I think i have found an issue on my s4.mini.LTE
with the official n.20191127 or with the chimpthechimp’s unofficial p.20191114, some times, i don’t understand why, I am stuck on a black screen with a blue or a red zone with date and clock on the top.
I don’t know how to get out of this screen, and I have to reboot the system.
It is probably just a screensaver !
But how to escape ?

Thank’s for replies.