I installed the Fedora 40 XFCE spin, and display does not blank, after the prescribed period of inactivity.
All F40 updates are installed. In Settings/Power Manager, under “Display” the “Display power management” is enabled, and all the knobs for both “On Battery” and “Plugged In” are set to reasonable values. However, whether plugged in or on battery the display does not blank after the prescribed period of inactivity.
I uninstalled xfce4-screensaver, this made no difference.