Comment 14 for bug 1697373

From what I can tell, Xwayland is still missing DPMS support:
https://cgit.freedesktop.org/xorg/xserver/tree/hw/xwayland/

And that's still what all X apps need to use. For example:
https://chromium.googlesource.com/chromium/src.git/+/master/services/device/wake_lock/power_save_blocker/power_save_blocker_x11.cc

This is a bug in Xwayland only. Xwayland needs to be modified to convert DPMS requests from clients like web browsers into calls to this:
https://cgit.freedesktop.org/wayland/wayland-protocols/tree/unstable/idle-inhibit