[CentOS] xfce on CentOS 7: can't lock screen

Ok, trying to bypass the usual flames (I used CDE for years, then
KDE until it got dumbed-down too much, and we all know how GNOME
has turned out) ... I've decided to try out xfce on CentOS 7.

I grabbed xfce from epel by installing the following via yum:
  epel-release
  @xfce

So far it's pretty good, and is giving me the basic features I'm looking
at without getting in my way.  However, I can't seem to get the damned
screen lock function to work.  

Leaving the monitor idle doesn't generally do anything. (Although just
now, after having left it for a few hours, I got a blank screen with
a few vertical narrow white bars that wouldn't respond to any key or
mouse movement.  Switching to a text console and back to the graphics
console, however, brought up the xfce desktop again, but with no 
screen lock prompt.)

If I select the 'Lock Screen' menu item from the top right corner 
of the screen it does nothing; doesn't lock the screen, provides
no feedback that there's a problem, etc.

The page
<http://xmodulo.com/control-screen-lock-settings-linux-desktop.html>
describes (for XFCE) bring up "Settings Manager" => "Screen Saver", but 
there is no "Screen Saver" option in the Settings Manager.

A clue stick would be appreciated.

Devin