psxrecomp/docs/GAME_MODE.md

Game Mode (Steam Deck, gamescope)

On a Steam Deck in Game Mode, or any gamescope session, there is no desktop. The launcher is an extra screen built for a mouse, and a windowed game sits inside a fullscreen compositor anyway. Game Mode therefore boots straight into the game, borderless fullscreen.

Source	Effect
--game-mode / --no-game-mode	on / off (wins over everything)
PSX_GAME_MODE=1 / =0	on / off
XDG_CURRENT_DESKTOP contains gamescope, or GAMESCOPE_WAYLAND_DISPLAY is set	on (detected)
SteamGamepadUI=1	on (detected Steam gamepad session)
none of the above	off: desktop behaviour, unchanged

Detection follows the active session. SteamDeck=1 identifies hardware and is also set in Desktop Mode; it does not enable Game Mode. Desktop sessions without gamescope or SteamGamepadUI=1 keep their usual launcher and window behavior. Use --no-game-mode (or PSX_GAME_MODE=0) to disable detection, or --launcher to reach the launcher for one run.

When it is on:

• With a disc already set up (settings.toml [disc] path, or --disc), the launcher is skipped and the game opens borderless fullscreen. The saved Fullscreen choice is not changed: a run where Game Mode is off keeps its own.
• On the first run, with no disc yet, the launcher opens so the player can pick their disc image. The next start goes straight in.
• --launcher still opens the launcher. In Game Mode, add it to the Steam shortcut's launch options to reach the settings.
• --headless and --hidden-window runs are never affected.

The runtime logs the decision, for example psxrecomp: game mode (gamescope session): fullscreen, no launcher.