A bare-spawn gamescope session is its own headless compositor, so it was the one
Linux route that never consulted effective_topology(): on a KDE desktop box the
physical panel kept showing the idle desktop for the whole stream while the
policy said exclusive. The KWin route's mechanism (disable the physicals) is
closed on this route — KWin refuses a configuration with zero enabled outputs
and no output on that desktop is ours to leave enabled — so the honest
translation is DPMS: the desktop stays exactly where it is, the panels go dark,
local input wakes them, and stream input never does (it enters gamescope's own
EIS socket, not KWin's libinput).
New kwin_dpms module drives the vendored org_kde_kwin_dpms protocol in-process
over the desktop's own Wayland (the kwin_output_mgmt stack and rationale), with
a kscreen-doctor --dpms fallback on kwin.rs's shared verdict/budget. The darken
is refcounted host-wide rather than floated through the registry's per-group
restore, because every gamescope spawn is its own group — the float alone would
re-light the panel when the first of two concurrent spawns ends. Each exclusive
spawn registers the release as its per-display topology restore, so the
registry still times every release (§6.1) and the last one out re-lights only
what the first darken actually turned off. Crash-safe by construction: DPMS is
non-persistent, so a dead host leaves nothing to journal — the panel re-lights
on the next local input.
Managed and Attach are deliberately untouched: managed's takeover already
stopped the desktop, and attach may be mirroring a gamescope that is itself
driving the physical panel.
enricobuehler