The laptop as a third monitor

The laptop can be the desktop’s third screen, right of DP-1, with the desktop’s own mouse and keyboard moving onto it and windows dragging across. A laptop’s HDMI port only outputs, so there is no cable that does this. Instead the desktop draws a monitor that does not physically exist, and streams it.

sequenceDiagram
    participant M as Moonlight (hutao-laptop)
    participant S as Sunshine (hutao-desktop)
    participant H as Hyprland (hutao-desktop)
    participant W as disconnect watcher

    M->>S: start "Laptop screen"
    S->>H: hyprctl output create headless LAPTOP
    Note over H: LAPTOP at 1920x0, 1920x1080@60
    S-->>M: wlr capture of LAPTOP, HEVC over the tailnet
    M--xS: disconnect (lid shut, wifi drop, window closed)
    S->>W: log line: CLIENT DISCONNECTED
    W->>S: POST /api/apps/close
    S->>H: hyprctl output remove LAPTOP

Using it

On the laptop: Moonlight → hutao-desktop → Laptop screen. To stop, close the stream or the lid; the screen goes away on its own.

Pairing is once per laptop, and it survives restarts. If it is ever lost (a reinstall, or ~/.config/sunshine wiped), pair from the laptop with a PIN of your choosing and give Sunshine the same PIN:

moonlight pair hutao-desktop --pin 1234     # on the laptop; waits

Then enter the PIN at https://localhost:47990 on the desktop, logging in as hutao with sunshine_password.

Moonlight’s defaults look bad for a desktop. Its default bitrate is tuned for games, and text smears at it. In Moonlight’s settings use 1080p, 60 FPS, about 40 Mbps and HEVC; the two machines have a direct LAN path over the tailnet. These live in Moonlight’s own settings file, which it rewrites, so they are not declared here. (2026-09-25)

Why it is built this way

• The monitor exists only while it is streamed. A permanent headless output would be an invisible screen the pointer wanders onto whenever the laptop is shut. Sunshine’s prep-cmd creates it on connect and removes it on quit.
• wlr capture, not KMS. A headless output has no CRTC, so there is nothing for KMS capture to read; output_name = "LAPTOP" picks it by name. The Selected monitor [DP-1] lines in the log before a stream are encoder probes from before the output exists, and are normal.
• No firewall ports. tailscale0 is trusted, so Moonlight reaches Sunshine over the tailnet and nowhere else. Avahi, which the Sunshine module turns on, is turned back off: mDNS never crosses the tailnet, so Moonlight adds the host by name.
• Audio stays on the desktop (stream_audio = disabled).
• The login is seeded from sops on every start. sunshine --creds merges into sunshine_state.json, so paired clients survive the re-seed.

The disconnect watcher

Sunshine only runs the undo half of a prep-cmd when the app quits. A plain disconnect keeps the app running so the client can resume, and that includes closing the lid, which leaves LAPTOP behind as the invisible screen this design exists to avoid. Sunshine has no disconnect hook. (2026-09-25)

So a user service bound to sunshine.service follows Sunshine’s journal for CLIENT DISCONNECTED and quits the app through Sunshine’s own API. Sunshine then runs its normal undo, its state stays consistent, and the next connect recreates the monitor. curl sends no Origin header, so the API wants basic auth (from the netrc) but no CSRF token.

It is keyed on that literal log line. If a Sunshine update rewords it, the watcher silently stops firing, and the symptom is an invisible screen right of DP-1 after the laptop disconnects. Check with:

journalctl --user -u sunshine | grep -E 'CLIENT DISCONNECTED|Executing Undo'

Each disconnect should be followed within a second by an Executing Undo Cmd. Until the watcher is fixed, hyprctl output remove LAPTOP clears the screen by hand.

A new user unit does not start on the switch that adds it. Sunshine is started by graphical-session.target, which is already up when you rebuild, and the watcher by Sunshine. The first time, run systemctl --user start sunshine sunshine-quit-on-disconnect, or log in again.