gnome - web browser as screensaver have problem with lock screen

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http://askubuntu.com – How to run a web browser as a screensaver From that link, I can now set chromium-browser in ubuntu - gnome in kiosk mode (fullscreen mode) to display a web page (acting as screensaver). Everything is working fine. But I want to setup it more security by adding a lock screen to ask user+password to unlock to user's desktop. gnome-screensaver or xscreensaver -lock can't be used here because if I run gnome-screensaver, xscreensaver lock screen, it will be in foreground of the screen, we can't see chromium-browser anymore. If we supply wrong password there, the gnome-screensaver is still in foreg (HowTos)