Hello @TooManyErrors, this issue is already being tracked at #1693. Closing as a duplicate.
And ... I'm going to disagree with myself. DPMS management and screensaver management should be tracked separately.
From what I can tell org.freedesktop.ScreenSaver universally controls idleness behaviors, including both DPMS and screensavers. I'll just close this issue, unless you think there still is a reason for this issue to be open.
Nothing extracted yet.
Your system information
Please describe your issue in as much detail as possible:
When a game is running, Steam should send screensaver inhibition signals to prevent systems from enabling screensavers, lock-screens, or entering sleep mode. This is particularly useful if someone is using a controller to play a game, as desktop environments may not monitor controller inputs when checking for idleness.
Utilizing the D-Bus service org.freedesktop.ScreenSaver (https://people.freedesktop.org/~hadess/idle-inhibition-spec/index.html) should solve this problem. Although it doesn't seem to be a formal standard, it is de-facto supported by (at least) KDE Plasma 5, GNOME 3, and XFCE.
There is a similar report at ValveSoftware/Proton#287 but no progress on a solution seems to have been made.