Created attachment 8683 Add support of xfce4-screensaver As xfce4-screensaver is fully functional, its support is missing. This patch fixes this issue.
To complement, removal of support for mate-screensaver would be a good idea. Mate-screensaver requires mate-power-manager, which conflicts with xfce4-power-manager, so it's pointless to support mate-screensaver unless user uses mate-power-manager instead of xfce4-power-manager. In my test machines mate-screensaver starts with xfce4-session (if it's installed) and does not let xfce4-scrrensaver do its job, and dows not allow to suspend/hibernate because mate-power-manager is not running. In this scenarios I had to uninstall mate-screensaver in way to use xfce4-screensaver properly (it works perfectly!).
Patch looks good to me. Applied at https://git.xfce.org/xfce/xfce4-power-manager/commit/?id=c06710f6ab925678b9130e680aa8c7014e9e75b4 As for removing support for mate-screensaver, I am a bit more hesitant since some users have likely come to rely on it in their workflows. If this is still desired, please create a new bug report for it.