Not OP but:
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on a desktop it's defaulted to desktop mode. I'm unsure about the steam deck.
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you choose. KDE or GNOME. Budgie is being worked on.
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lutris can install your windows executables. Bottles is available too.
The only games I'm unable to play so far have been AAA games with unfriendly anticheat. ProtonDB helps here.
How long before Nintendo slaps a DMCA on this one?