Third compositor on the VirtualDisplay seam. gamescope's model differs from KWin/Mutter: it's
not a runtime protocol but a micro-compositor we spawn — `gamescope --backend headless -W -H -r
-- <app>` — which composites at the client's size AND refresh natively (so no separate
refresh step), runs the app nested, and exports a built-in PipeWire node named "gamescope".
The backend spawns it, discovers that node via pw-dump, and returns a VirtualOutput whose
keepalive owns the process (drop = kill = teardown). App via LUMEN_GAMESCOPE_APP. Select with
LUMEN_COMPOSITOR=gamescope; m0's virtual source now honors LUMEN_COMPOSITOR so any backend is
testable without a client. Input (gamescope's libei/EIS socket) is a follow-up.
Builds/clippy/fmt clean. Needs gamescope installed to validate; headless capture on the
proprietary NVIDIA driver is plausible-by-architecture but unproven — validate empirically.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
enricobuehler
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