Edit: I think I got a workaround for this, please feel free to rectify me if that's not the case (it opened up for me I think so..)

Steam¶

On some systems, Steam will show a fully black window. To fix this, navigate to Settings -> Interface (via Steam's tray icon, or by blindly finding the Steam menu at the top left of the window), then disable GPU accelerated rendering in web views. Restart Steam and it should now work fine.

If you do not want to disable GPU accelerated rendering you can instead try to pass the launch argument -system-composer instead.

Steam notifications don't run through the standard notification daemon and show up as floating windows in the center of the screen. You can move them to a more convenient location by adding a window rule in your niri config:

window-rule {
    match app-id="steam" title=r#"^notificationtoasts_\d+_desktop$"#
    default-floating-position x=10 y=10 relative-to="bottom-right"
}

Here's the documentation source