Brand-new Rpi3B+ with a brand-new download of Stretch.
Made sure everything is up to date:
sudo apt-get update && sudo apt-get upgrade -y sudo apt-get autoremove -y sudo apt-get dist-upgrade -y
I don't mind the automatic screen blanking but would prefer that it ask for a login password when it wakes up. In Jessie that is easy.
So I went through the normal Jessie-style installation of screensaver controls:
sudo apt-get update sudo apt-get install xscreensaver xscreensaver-gl-extra xscreensaver-data-extra -y
And then, in Preferences, there is new a Screensaver option.
HOWEVER, when I click on it, it blinks as it starts up, then simply goes away.
So, to properly remove it, I used:
sudo apt-get purge xscreensaver xscreensaver-gl-extra xscreensaver-data-extra -y sudo apt-get autoremove -y