docs: known limitations + follow-ups for the session-aware host

Capture the deliberately-parked items after live-validating the session-aware
backend selector on the Bazzite F44 box (Desktop KDE + Gaming both at the
client's resolution, warm reuse, Feature B mid-stream switch both directions).

Top follow-ups: (1) F44 gamescope teardown corrupts the GPU context (try SIGKILL
teardown, else keep the managed session warm); (2) mid-stream-switch input is
flaky until a reconnect (portal opens before the systemd/D-Bus activation env
settles — fix: import-environment on switch); (3) the KWin virtual output isn't
set primary. Plus polish: input-loss window on switch, the recovered NVENC
invalid-param log, the 4090 HEVC ~800Mbps cap, restore-guard/keep-warm
interaction, and promoting Feature B from opt-in to default.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/session-aware-host-followups.md

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+# Session-aware host — known limitations & follow-ups
+
+Status: 2026-06-14. The host auto-detects the live session (Gaming / KDE / GNOME / wlroots) **per
+connect** and routes both video and input at it — managed gamescope at the client's resolution in
+Steam Gaming Mode, a KWin/Mutter virtual output at the client's resolution on a Desktop. A watcher
+(opt-in: `PUNKTFUNK_SESSION_WATCH=1`) follows a Gaming↔Desktop switch **mid-stream** and rebuilds the
+backend in place without a reconnect.
+
+Live-validated on the Bazzite F44 box (`bazzite-deck-nvidia:testing`, RTX 4090): Desktop KDE at
+5120×1440 + input; Gaming managed at 5120×1440; warm-session reuse on quick reconnect; Feature B
+video-switch both directions. The items below are **deliberately parked** — they have workarounds
+and/or are F44-specific.
+
+## High priority
+
+### 1. F44 gamescope teardown corrupts the GPU context
+Every gamescope teardown on this box (stop the autologin on connect; stop the managed session on
+restore) risks leaking the NVIDIA GPU context — surfaces as `CUDA_ERROR_ILLEGAL_STATE` (401) in
+`cuCtxCreate` / `vkCreateDevice` `VK_ERROR_INITIALIZATION_FAILED` (-3), then a black screen that
+**needs a reboot**. The 5 s debounced restore + the desktop restore-guard cut the teardown *count*
+but don't eliminate it. Options, in order of preference:
+- **SIGKILL the gamescope on teardown** instead of `systemctl stop` (SIGTERM). Hypothesis: skipping
+  gamescope's buggy SIGTERM teardown handler (the part that SIGSEGVs, exit 139) lets the process die
+  hard and the driver reclaim its GPU resources cleanly via normal process exit — no half-torn-down
+  context. Change `stop_autologin_sessions` + `stop_session` (`vdisplay/gamescope.rs`) to
+  `systemctl --user kill --signal=SIGKILL <unit>` (+ a follow-up `stop`/`reset-failed` to clear unit
+  state). **Untested** — this is the first thing to try; it would preserve "managed client-res
+  gaming AND TV-shows-gaming-when-idle".
+- **Keep the managed session warm** (no per-disconnect restore): spawn once, reuse forever, never
+  tear down → ~1 teardown per host lifetime. Tradeoff: the TV is blank/idle when no client is
+  connected (the autologin is never restored; return to gaming manually).
+- Upstream gamescope/driver fix.
+
+### 2. Mid-stream-switch input (Gaming→Desktop) is flaky until a reconnect
+After a mid-stream switch to a desktop, pointer/keyboard often don't land until a disconnect+reconnect.
+Root cause: on the switch the host opens the KDE `RemoteDesktop` **portal** immediately, but the
+portal is a D-Bus-activated service that reads the **systemd `--user` activation environment**, which
+hasn't settled to the new session yet — so the portal session is created against a half-stale env. It
+*accepts* events (no error, so the injector's reopen-on-failure never fires) but they don't reach
+KDE. The host log even shows `libei: portal granted devices` + `device RESUMED`, yet input is dead.
+**Workaround: reconnect once after switching** (re-opens the portal against the settled env → works).
+Fix: on a session switch, push the new session env into the systemd/D-Bus activation environment
+before reopening input — `systemctl --user import-environment WAYLAND_DISPLAY XDG_CURRENT_DESKTOP
+DBUS_SESSION_BUS_ADDRESS` + `dbus-update-activation-environment` (exactly what a fresh KDE login does;
+see `scripts/headless/run-headless-kde.sh:118-119`) — and/or delay/retry the input reopen until the
+portal is settled.
+
+### 3. KWin virtual output is not set as primary
+The KWin virtual output isn't marked the primary display, so KDE panels / primary-screen content can
+stay on the (now-absent) original output instead of the streamed virtual screen. Follow-up: set the
+virtual output primary on create — `vdisplay/kwin.rs` already reads `PUNKTFUNK_KWIN_VIRTUAL_PRIMARY`;
+make it default-on (or always promote the new output to primary via the KWin/kscreen API) so the
+streamed screen is the primary desktop.
+
+## Lower priority / polish
+
+### 4. Mid-stream-switch input loss window (~6 s)
+During the libei portal setup on a switch, buffered input drops (`libei: DROP — no resumed device`,
+hundreds of events). Polish: pre-warm the portal, or hold events instead of dropping during the
+device-resume window.
+
+### 5. NVENC `InitializeEncoder failed: invalid param` (recovered)
+At 5120×1440@240 the first NVENC open fails with `invalid param (8)` and **recovers** via the 2-way
+split-encode path (the stream is live). Cosmetic but noisy — investigate the first-attempt failure /
+silence the log.
+
+### 6. NVENC HEVC bitrate cap (~800 Mbps on the RTX 4090)
+HEVC opens at the GPU's max (~800 Mbps) when a higher rate is requested (e.g. 1600). Not a bug;
+consider preferring AV1 when the client requests >~800 Mbps HEVC, and surface the cap in the
+speed-test / bitrate UI.
+
+### 7. Restore-guard / keep-warm model interaction
+`do_restore_tv_session`, when a desktop is active, still stops the idle managed gamescope (a teardown
+— leak risk per #1) and consumes `STOPPED_AUTOLOGIN` (so a later return-to-gaming won't auto-restore
+the TV session). Resolve together with the keep-warm decision in #1.
+
+### 8. Feature B is opt-in
+The mid-stream watcher is gated behind `PUNKTFUNK_SESSION_WATCH=1` pending broader validation. Promote
+to default-on once #2 (mid-stream input) lands and it's exercised on more boxes.