ducoterra
8fefd83ef8
An unattended wall failed in two ways on 2026-11-02. It hard-hung with the kernel alive but userspace unable to start a process: ssh completed the handshake and closed 6-13 ms later with zero bytes and no banner, while 62 GiB RAM and zero OOM lines rule out memory. Separately, the service knew at 14:03Z that nothing was watching (healthz viewers:0) and nothing was reading it. It was discovered by walking over to the panel. Hang recovery, two independent layers (install step 7/9, asserted by check-kiosk.sh): - /etc/sysctl.d/90-mirror-wall-selfheal.conf: soft lockup, hard lockup and any oops escalate to a panic, and kernel.panic=30 reboots instead of parking. - /etc/systemd/system.conf.d/90-mirror-wall-watchdog.conf: PID 1 must keep kicking the iTCO hardware watchdog or the chipset resets the box, which is the layer that covers a kernel too wedged to notice. Clamped to the driver's own 30s because systemd kicks at a third of whatever value it is handed. Presence alerting: docs/wall-watch/ runs on the server host, probes loopback /healthz (exempt from the address gate) every minute, debounces three checks, pages over ntfy and then announces the recovery. WALL_DRILL=1 labels a test page in the title, body and priority so a drill can never be mistaken for an incident. Verified against a fake ntfy: caller environment wins over the config file, both transitions fire, and the debounce holds. Firewall (install step 8/9): drop FedoraWorkstation's 1025-65535/tcp. The UDP half is load-bearing for peer-to-peer media (mirror deliberately uses no STUN/TURN) and stays open; the TCP half was every high port on a box whose only access control is its address. Measured, not assumed: a live share kept flowing at 459.6 kbit/s inbound with the TCP range closed, and tcp/8080 stopped answering from a same-subnet host. Also fixed: install-kiosk.sh overwrote an existing client's kiosk.conf with its built-in default URL, so re-running it to repair a wall silently repointed it at the production default. An existing URL now wins and KIOSK_URL= is an explicit change. Documented with the evidence: why gnome-remote-desktop cannot share a gnome-kiosk session (gnome-kiosk exports no org.gnome.Mutter.* names, so grd has nothing to capture, and this image has grdctl rather than grdc), and why no userspace RDP would have survived the hang either. LLM-Generated: true
207 lines
11 KiB
Markdown
207 lines
11 KiB
Markdown
# mirror
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**Device-agnostic screen mirroring that lives entirely in the browser.**
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`mirror` runs a small HTTP server on your machine. Open it from any device on
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your network:
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| Page | URL | What it does |
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| ------------ | ------------------ | ----------------------------------------------------------------------- |
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| **Share** | `/mirror` (also `/share`) | Picks a window, screen, or tab via the OS-native picker and mirrors it. |
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| **Watch** | `/watch` | Shows every screen currently being mirrored; click a tile to take the stage. |
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| **Index** | `/` | Signpost + live count of mirrors. |
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Any number of people can mirror at the same time and any number of people can
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watch. There are **no accounts and no passwords** — this is built for a trusted
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local network, and that assumption is deliberate and documented in
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[SECURITY.md](SECURITY.md). What *does* decide who gets in is the source address:
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by default any address is welcome, and `--allow-cidr` / `MIRROR_ALLOW_CIDRS`
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narrows that to the networks you name.
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## Quickstart
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```bash
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uv sync --extra dev
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uv run mirror serve # http://0.0.0.0:8055
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```
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Then, from another machine on the LAN:
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```
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http://<this-machine>:8055/mirror # to share
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http://<this-machine>:8055/watch # to watch
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```
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Linux is the primary target. On Linux the native picker is X11/Wayland's own
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(Chromium uses `xdg-desktop-portal` on Wayland/GNOME/KDE, and its built-in X11
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picker on Xorg), so nothing has to be installed on the client side.
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The share page works on a phone as well as a desktop: `/mirror` is usable
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down to a 320 px-wide viewport, and the preview keeps its shape but never
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shrinks below the controls it holds, so the Share button is always on screen.
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The target *client* setup for an always-on display is Fedora Silverblue running
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GNOME Kiosk — see [`docs/kiosk-silverblue.md`](docs/kiosk-silverblue.md).
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## How it works
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Media never touches the server. Each mirror is a WebRTC peer connection
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established directly between the sharer's browser and each watcher's browser
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(a "mesh" fan-out). The server is a **signaling relay and roster only**: it
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tracks who is publishing what, forwards SDP/ICE between peers, and pushes
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roster deltas to viewers.
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```
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sharer browser ──ws──► ┌──────────────────────┐ ◄──ws── watcher browser
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│ │ FastAPI │ ▲
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│ offer/ICE ───►│ /ws (signalling) │◄─── offer/ICE │
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│ │ registry (in-mem) │ │
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└────── WebRTC (SRTP, LAN, p2p) ─────────┴─────────────────┘
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```
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## Configuration
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Environment variables (all optional; a `.env` file is loaded if present):
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| Variable | Default | Meaning |
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| ---------------------- | ------- | ---------------------------------------------------- |
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| `MIRROR_HOST` | `0.0.0.0` | Bind address for `mirror serve`. |
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| `MIRROR_PORT` | `8055` | Bind port. |
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| `MIRROR_ICE_SERVERS` | `[]` | JSON array of RTCIceServer objects, if peers cannot reach each other directly. |
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| `MIRROR_MAX_MIRRORS` | `0` | Cap on concurrent mirrors (`0` = unlimited). |
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| `MIRROR_MAX_BITRATE` | `25000000` | Per-leg WebRTC bitrate cap in bits/s — the mesh uploads N × bitrate per sharer, so this is the ceiling. 25 Mbit/s suits a wired LAN at 1440p; a Wi-Fi sharer with many viewers may need less, not more. |
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| `MIRROR_PING_INTERVAL` | `20` | Seconds between socket liveness pings. |
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| `MIRROR_ALLOW_CIDRS` | *(unset)* | Comma-separated source networks allowed to connect, e.g. `10.2.0.0/22,fd00::/64`. Unset means unrestricted. A wall needs this range too, and its address is DHCP — give it a reservation. |
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| `MIRROR_TRUSTED_PROXIES` | `127.0.0.0/8, ::1/128` | Which peers may speak for a client via `X-Forwarded-For`. Only a proxy that overwrites the header belongs here. |
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| `DEBUGPY` | `0` | `1` opens a debugpy listener on `DEBUGPY_PORT`. |
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| `DEBUGPY_PORT` | `5678` | debugpy port. |
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| `DEBUGPY_WAIT` | `0` | `1` blocks the server at start until a debugger attaches. |
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## Platform notes
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**Screen capture needs a secure context.** `http://localhost` is one; a plain
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`http://192.168.x.x` opened from another device is not, and the browser hides
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`getDisplayMedia()` completely. The `/mirror` page detects this and prints the
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three ways forward (share from localhost, serve HTTPS, or mark that one origin as
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secure on the sharing device). Watching never needs permission.
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This is why the fleet deployment terminates TLS in nginx rather than exposing
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`mirror serve` directly — over `https://`, every device on the network can share
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and none of this applies. A bare `mirror serve` is for the machine it runs on and
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for testing.
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**Linux.** On X11/Xorg Chromium uses its native capturer — the picker appears and
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that is the end of it. On Wayland (GNOME/KDE) Chromium goes through
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`xdg-desktop-portal`, so the compositor shows its **own permission dialog for
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each share**, which mirror neither controls nor tries to bypass. Chrome's picker
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offers whole screens and individual windows; on Wayland what the portal will
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agree to depends on the compositor.
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**Audio.** A share carries audio when the browser's picker offers it (Chrome's
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"share audio" tickbox, or a tab's own audio); the sharer's panel says
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`shared` / `silent` / `not offered`, and the wall shows an audio button only
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when one exists. On the wall, only the staged mirror can be audible — **press
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`m` on the wall to hear the staged mirror** (unmuting it mutes the others,
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leaving the stage re-mutes it). What *system* audio a browser can capture is
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a browser/platform fact, not a mirror one: Chrome on Linux gets it where
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PipeWire offers it, Firefox's desktop capture does not offer it at all. See
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[SECURITY.md](SECURITY.md) §4 "Audio".
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## Deployment
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The internal deployment is `https://mirror.reeseapps.com`, described and automated
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in the deployments repo under `mirror/` (`app/` quadlet, `nginx/` edge, `certbot/`,
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`route53/`, and a README with the VM SOP). Three things about it are worth knowing
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from this side:
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- **TLS ends at nginx on the service VM**, not in this process, so there is no
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`--https` flag here. That is what makes `getDisplayMedia()` available on every
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device, and it means an `Upgrade` header and a long idle timeout on `/ws` are
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load-bearing.
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- **The address list is enforced twice** — nginx `allow`/`deny`, and
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`MIRROR_ALLOW_CIDRS` in the container for the path that bypasses nginx. Change
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both in one commit.
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- **The container has no internet egress.** Media goes browser-to-browser and the
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server only relays JSON, so it dials nothing; `Internal=true` is what makes
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that a fact rather than an intention.
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### Kiosk client
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A wall display is a Fedora Silverblue box running the locked-down GNOME Kiosk
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session, auto-logged in as `kiosk`, with Firefox in `--kiosk` mode pointed at
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`/watch`. It is set up and repaired entirely over SSH — no keyboard, no console,
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no monitor. A brand-new box layers the kiosk packages once first
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(`rpm-ostree install gnome-kiosk gnome-kiosk-script-session`, then a reboot);
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after that these two commands are both the setup and the repair path:
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```bash
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scp -r docs/kiosk-silverblue client:/var/tmp/ && ssh client \
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'cd /var/tmp/kiosk-silverblue && sudo sh install-kiosk.sh && sudo systemctl restart gdm'
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ssh client 'sudo sh /usr/local/share/mirror-kiosk/check-kiosk.sh'
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```
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Because nobody is standing at the machine, every claim about it is verified from
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the outside: `loginctl` for the session, the foreground VT for "is the wall
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actually on screen", the server's `/api/streams` for the viewer, the sharer's own
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peer state for the media, and a probe page for the rendered resolution. The
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runbook records why each file lives where it does on an
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otherwise read-only OS.
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**When it hangs.** A wall is unattended, so a hang has to end without a person:
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the kernel turns lockups and oopses into a reboot, and PID 1 must keep kicking the
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hardware watchdog or the chipset resets the box. Being *quiet* is a different
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failure than being hung, so the server watches `/healthz` for that and pages over
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ntfy ([`docs/wall-watch/`](docs/wall-watch/)). Both are installed by the commands
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above and asserted by `check-kiosk.sh`.
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**Keys.** The whole wall works from a keyboard — and the box needs no keyboard
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attached to be operated *or* debugged; the keys are there when one is plugged
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in. On `/watch`:
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| Key | Action |
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| --- | --- |
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| `j` / `↓` · `k` / `↑` | Select the next / previous mirror (wraps). |
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| `Enter` / `f` | Stage the selected mirror fullscreen (`f` re-enters fullscreen when already staged). |
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| `g` | Back to the wall. |
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| `n` / `p` | Stage the next / previous mirror. |
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| `1`–`9` | Stage the Nth mirror. |
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| `m` | Mute / unmute the staged mirror — on the selected mirror it stages it first. |
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| `?` | Show / hide this list on screen. |
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| `Esc` | Close the key list, then leave the stage. |
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A mirror's audio is only heard while it is on the stage: every mirror starts
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muted, unmuting one (the tile's audio button or `m`) mutes the others, and
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leaving the stage re-mutes it. Sound never starts without a click or a
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keypress.
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On `/mirror`: `s` starts a share, `x` stops one, `Esc` stops while live. Keys
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with `Ctrl`/`Cmd`/`Alt` are never intercepted, and a key typed in the display-
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name field is text, not a command. `?` is the only key that needs Shift, and it
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is matched on the character itself — on a layout where `?` is a dead key the
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list is not reachable, so keep this table to hand.
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## Development
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```bash
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uv run pytest # unit + API + browser E2E (~50s)
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uv run pytest -m "not e2e" # everything except the browser (~0.5s)
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uv run ruff check . && uv run ruff format --check . && uv run pyright
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```
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The browser suite drives **real capture**: Chromium's fake media device yields a
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moving monitor stream (1280×720 by default; clamped to the capturing window when
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a size is requested, as `/mirror` now asks), so the assertions are on decoded,
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changing pixels at the watcher — not on log lines.
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There is a second, opt-in rung that exercises Chromium's **real X11 capture
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backend** on a private Xvfb display, isolated from your own session so it can
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never prompt you or mirror your desktop:
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```bash
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MIRROR_REALDISPLAY=1 uv run pytest -m realdisplay
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```
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See [AGENTS.md](AGENTS.md) for the operating rules,
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[.agents/PLAN.md](.agents/PLAN.md) for the locked architectural decisions, and
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[SECURITY.md](SECURITY.md) for why there is no login.
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