[SOLVED] [Debian Testing with Mate]: screen flick and cursor and focus lost on login screen

Hi all
After last system update I experience two annoying behaviors.
But this thread is about first of them:
Now the GUI login screen always refreshes after a couple of seconds and - which is worse - login dialog window loses its focus. This is very annoying - I have to wait and start typing after this ugly flick.
Any Ideas how to dig for core reason an try to repair?

P.S: I don't use any external graphics accelerator. Intel HD Graphhics only.

regards

Ok, problem solved. I had to change acceleration method from "SNA" to old "UXA" in /usr/share/X11/xorg.conf.d/20-intel.conf.
Strange, since it was working perfectly with SNA for a long time.