Just found out there’s a steamos-nested-desktop command in SteamOS 3.5 and someone was asking about changing the resolution since it defaults to 1280x800.

That script is actually the one posted by a KDE dev in a thread some time last year, line for line. I was able to get it to change resolution on start by adding a couple lines before the cat < $XDG_RUNTIME_DIR/nested_plasma/kwin_wayland_wrapper

to set an X and Y variable that uses xrandr to parse your display’s current maximum resolution and sets them to the respective variables. Then you can change the line:

/usr/bin/kwin_wayland_wrapper --width 1280 --height 800 --no-lockscreen \$@

to:

/usr/bin/kwin_wayland_wrapper --width $X --height $Y --no-lockscreen \$@

so that running the script will set KDE’s resolution dynamically. This will automatically scale the desktop to whatever resolution your primary monitor is at without having to change anything.

You can also set the resolution from Gamemode by going to properties > Game Resolution and changing it to whatever you actually want. Gamescope will present a virtual display to KDE and tell it your chosen resolution. I’ve been using this since I found that dev’s post and it’s been handy. I can use Steam’s own passcode system as a “lockscreen” since the SDDM one is disabled in SteamOS.

Should look like this with the changes above

Hope this helps whoever’s looking for this.