The host cursor stops riding the video and becomes a real OS cursor on
the client (the Parsec/RDP model): pointer feel no longer pays the
capture→encode→network→decode→present round trip.
Wire (M2a):
- Hello grows a client_caps trailing byte (CLIENT_CAP_CURSOR) after the
fixed display_hdr block — presence disambiguated by remaining length,
which caps the post-HDR tail at 27 bytes (documented); Welcome answers
HOST_CAP_CURSOR (capable-and-asked, the 444/clipboard precedent).
- CursorShape (0x50, control stream): serial + dims + hotspot + straight
RGBA, ≤120px/side so the u16 frame always fits (128² would overshoot);
client caches by serial — re-showing a known shape costs 14 bytes, not
a bitmap (RDP pointer-cache for free).
- CursorState (0xD0 datagram): serial + visible/relative_hint flags +
position, sent once per encode-loop tick — latest-wins, self-healing
under loss, no refresh timer. relative_hint is reserved for M3.
- Client core: two new planes (control-task + datagram-task arms) →
next_cursor_shape/next_cursor_state; connect() grows client_caps
(C ABI passes 0 until the v11 cursor poll fns exist).
Host (M2b, Linux portal only):
- handshake::cursor_forward is THE predicate (client asked ∧ Linux ∧
compositor ≠ gamescope) — Welcome bit and session wiring both read it.
- SessionPlan.cursor_blend goes false for a forwarding session; the
encode loop ticks a CursorForwarder every iteration: shape-serial diff
→ control-task bridge (mirrors probe_result), state datagram → conn.
- CursorOverlay/capture CursorState carry the hotspot through
(nearest-neighbor downscale backstop for XL cursors, unit-tested).
Presenter:
- CursorChannel drains both planes per loop iteration; shapes become
SDL color cursors (from_surface + hotspot), applied while the desktop
mouse model is engaged; visibility follows the host; capture/released
hands back the system cursor. Sessions advertise the cap when they
START in desktop mode.
Verified on .21: fmt + clippy -D warnings (7 crates) + tests green
(core 218 incl. new wire roundtrips, host 245 incl. e2e + forwarder
downscale tests).
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
