ci: run stream-recycle on every change

It failed on main before this PR: the media cache leaked one blob per
slide per refresh until the browser refused the display's own requests.
With one object URL per slide it holds three live URLs for three slides,
and CI now keeps it there, together with "stays live for the whole watch".

Co-Authored-By: Claude Opus 5.5 <[email protected]>

.gitea/workflows/ci.yml

@@ -0,0 +1,197 @@
+name: CI
+
+# Signage builds, tests, and runs only inside an Ekklesia workspace, so every job first assembles it
+# into the platform commit pinned in ci/platform.env (ci/assemble.sh) and installs that workspace
+# with --frozen-lockfile from ci/pnpm-lock.yaml.
+#
+# The same trust rules as the platform's own CI (docs/development/security.md there):
+#   * every action is pinned by a full commit SHA;
+#   * every checkout sets `persist-credentials: false`, and nothing in this workflow pushes;
+#   * no repository, organization, or instance secret is referenced;
+#   * there is no `pull_request_target`;
+#   * the self-hosted runner executes trusted, reviewed code only.
+#
+# The server is Gitea 1.24, which ignores `permissions`, `concurrency`, and `timeout-minutes`
+# (https://docs.gitea.com/1.24/usage/actions/comparison). They are declared so the workflow is already
+# correct on a release that honours them; on 1.24 they bound nothing, and `permissions` does not
+# restrict the job token.
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ci-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  lint:
+    name: lint
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          # Nothing here pushes, so the job token is never needed after checkout and must not be left
+          # in .git/config for a later step to read.
+          persist-credentials: false
+
+      - name: Assemble Signage into the pinned platform
+        run: bash ci/assemble.sh
+
+      - name: Set up Node.js at the platform's version
+        uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6.4.0
+        with:
+          node-version-file: build/workspace/.node-version
+
+      - name: Install the assembled workspace (frozen lockfile)
+        working-directory: build/workspace
+        run: |
+          corepack enable
+          corepack install
+          pnpm install --frozen-lockfile
+
+      - name: Lint
+        working-directory: build/workspace
+        run: pnpm --filter @ekklesia/signage lint
+
+  typecheck:
+    name: typecheck
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          # Nothing here pushes, so the job token is never needed after checkout and must not be left
+          # in .git/config for a later step to read.
+          persist-credentials: false
+
+      - name: Assemble Signage into the pinned platform
+        run: bash ci/assemble.sh
+
+      - name: Set up Node.js at the platform's version
+        uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6.4.0
+        with:
+          node-version-file: build/workspace/.node-version
+
+      - name: Install the assembled workspace (frozen lockfile)
+        working-directory: build/workspace
+        run: |
+          corepack enable
+          corepack install
+          pnpm install --frozen-lockfile
+
+      - name: Build the platform packages Signage depends on
+        working-directory: build/workspace
+        run: pnpm exec turbo run build --filter='@ekklesia/signage^...'
+
+      - name: Typecheck the server, browser, and browser-test projects
+        working-directory: build/workspace
+        run: pnpm --filter @ekklesia/signage typecheck
+
+  test:
+    name: test
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          # Nothing here pushes, so the job token is never needed after checkout and must not be left
+          # in .git/config for a later step to read.
+          persist-credentials: false
+
+      - name: Assemble Signage into the pinned platform
+        run: bash ci/assemble.sh
+
+      - name: Set up Node.js at the platform's version
+        uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6.4.0
+        with:
+          node-version-file: build/workspace/.node-version
+
+      - name: Install the assembled workspace (frozen lockfile)
+        working-directory: build/workspace
+        run: |
+          corepack enable
+          corepack install
+          pnpm install --frozen-lockfile
+
+      - name: Build the platform packages Signage depends on
+        working-directory: build/workspace
+        run: pnpm exec turbo run build --filter='@ekklesia/signage^...'
+
+      - name: Unit, browser, and display contract tests
+        working-directory: build/workspace
+        run: pnpm --filter @ekklesia/signage test
+
+  build:
+    name: build
+    runs-on: ubuntu-latest
+    timeout-minutes: 20
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          # Nothing here pushes, so the job token is never needed after checkout and must not be left
+          # in .git/config for a later step to read.
+          persist-credentials: false
+
+      - name: Assemble Signage into the pinned platform
+        run: bash ci/assemble.sh
+
+      - name: Set up Node.js at the platform's version
+        uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6.4.0
+        with:
+          node-version-file: build/workspace/.node-version
+
+      - name: Install the assembled workspace (frozen lockfile)
+        working-directory: build/workspace
+        run: |
+          corepack enable
+          corepack install
+          pnpm install --frozen-lockfile
+
+      # The same gates the platform's image build runs against exactly this module set: its boundary
+      # rules (which apply to modules), the manifest, and the migrations.
+      - name: Platform gates (boundaries, module manifest, migrations)
+        working-directory: build/workspace
+        run: |
+          pnpm boundaries
+          pnpm modules:validate
+          pnpm migrations:validate
+
+      - name: Build Signage and the platform it depends on
+        working-directory: build/workspace
+        run: pnpm exec turbo run build --filter='@ekklesia/signage...'
+
+  # Signage inside Ekklesia, end to end, in a real browser: the platform's own Dockerfile builds the
+  # deployment image with Signage in it, and the platform's split-topology deployment smoke runs
+  # against it, pairing a kiosk display and running the scenarios in ci/display-scenarios. A
+  # disposable database and object store with generated secrets; nothing is pushed or logged in to.
+  integration:
+    name: integration
+    needs:
+      - test
+      - build
+    runs-on: ubuntu-latest
+    timeout-minutes: 45
+    env:
+      DOCKER_BUILDKIT: "1"
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          persist-credentials: false
+
+      - name: Assemble Signage into the pinned platform
+        run: bash ci/assemble.sh
+
+      - name: Build the image and run the display scenarios
+        run: bash ci/integration.sh

.gitignore

@@ -4,3 +4,4 @@ dist/
 coverage/
 *.log
 .DS_Store
+build/

ci/assemble.sh

@@ -0,0 +1,72 @@
+#!/usr/bin/env bash
+# Assembles Signage into the Ekklesia platform it is tested against: the platform at the commit pinned
+# in ci/platform.env, this checkout at modules/signage, and ci/pnpm-lock.yaml as the workspace
+# lockfile. Signage cannot build or run outside an Ekklesia workspace, so every CI job starts here.
+#
+#   bash ci/assemble.sh [--allow-missing-lock] [output directory]
+#
+# The output directory (default build/workspace) is deleted and recreated. The platform is fetched
+# by its `main` branch and the pinned commit must be on it, so CI only ever runs reviewed platform
+# code; the checkout is then by commit, so nothing after it depends on a ref the network can move.
+# Every .git directory is removed afterwards.
+#
+# --allow-missing-lock keeps the platform's own lockfile. Only ci/update-lock.sh uses it.
+set -Eeuo pipefail
+
+root="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")/.." && pwd)"
+allow_missing_lock=0
+if [[ "${1:-}" == "--allow-missing-lock" ]]; then
+  allow_missing_lock=1
+  shift
+fi
+out="${1:-$root/build/workspace}"
+
+fail() {
+  echo "assemble: $*" >&2
+  exit 1
+}
+
+for command in git tar; do
+  command -v "$command" >/dev/null || fail "$command is required."
+done
+
+# Read the two pins without sourcing the file: it is data, not a script.
+pin() {
+  local value
+  value="$(sed -n "s/^$1=//p" "$root/ci/platform.env")"
+  [[ -n "$value" ]] || fail "ci/platform.env has no $1."
+  printf '%s' "$value"
+}
+repository="$(pin EKKLESIA_PLATFORM_REPOSITORY)"
+commit="$(pin EKKLESIA_PLATFORM_COMMIT)"
+[[ "$commit" =~ ^[0-9a-f]{40}$ ]] || fail "EKKLESIA_PLATFORM_COMMIT must be a full commit SHA."
+
+rm -rf "$out"
+mkdir -p "$(dirname -- "$out")"
+git init --quiet "$out"
+git -C "$out" remote add origin "$repository"
+git -C "$out" fetch --quiet --no-tags origin "+refs/heads/main:refs/remotes/origin/main" ||
+  fail "cannot fetch the platform's main from ${repository}."
+git -C "$out" cat-file -e "${commit}^{commit}" 2>/dev/null ||
+  fail "the pinned platform commit ${commit} is not on the platform's main."
+git -C "$out" merge-base --is-ancestor "$commit" refs/remotes/origin/main ||
+  fail "the pinned platform commit ${commit} is not on the platform's main."
+git -C "$out" -c advice.detachedHead=false checkout --quiet --detach "$commit"
+[[ "$(git -C "$out" rev-parse HEAD)" == "$commit" ]] || fail "the platform did not check out at ${commit}."
+rm -rf "$out/.git"
+echo "assemble: platform ${commit}"
+
+[[ ! -e "$out/modules/signage" ]] || fail "the platform checkout already has modules/signage."
+mkdir -p "$out/modules/signage"
+# Exactly the files this checkout tracks, as committed or as changed in the working tree. Nothing
+# installed, built, or ignored, and not the CI scaffolding, which is not part of the module.
+git -C "$root" ls-files -z --cached --others --exclude-standard -- . ':!ci' ':!.gitea' |
+  tar -C "$root" --null --files-from=- --create --file=- |
+  tar -C "$out/modules/signage" --extract --file=-
+echo "assemble: signage $(git -C "$root" rev-parse HEAD 2>/dev/null || echo "(working tree)")"
+
+if [[ -f "$root/ci/pnpm-lock.yaml" ]]; then
+  cp "$root/ci/pnpm-lock.yaml" "$out/pnpm-lock.yaml"
+elif ((allow_missing_lock == 0)); then
+  fail "ci/pnpm-lock.yaml is missing; run bash ci/update-lock.sh."
+fi

ci/display-scenarios

@@ -0,0 +1,7 @@
+# The platform's display-device scenarios this repository's CI runs against Signage, one per line
+# (scripts/lib/deploy-smoke-display.mjs in the pinned platform). Pairing a kiosk and restarting it on
+# the same profile always run first. A change that makes another scenario pass adds it here.
+two-pages
+session-blip
+stream-failure
+stream-recycle

ci/integration.sh

@@ -0,0 +1,38 @@
+#!/usr/bin/env bash
+# Signage inside Ekklesia, end to end: builds the deployment image from the assembled workspace with
+# the platform's own Dockerfile, then runs the platform's split-topology deployment smoke against it
+# with the display-device scenarios listed in ci/display-scenarios. A disposable PostgreSQL and object
+# store, generated secrets, and an explicit migration apply against that fresh database only; nothing
+# is pushed, published, or logged in to. The image is removed afterwards.
+#
+#   bash ci/integration.sh            (after bash ci/assemble.sh)
+set -Eeuo pipefail
+
+root="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")/.." && pwd)"
+workspace="$root/build/workspace"
+[[ -f "$workspace/deploy/Dockerfile" ]] || {
+  echo "integration: run bash ci/assemble.sh first." >&2
+  exit 1
+}
+
+image="signage-ci:${GITHUB_SHA:-local}-${GITHUB_RUN_ID:-0}-${GITHUB_RUN_ATTEMPT:-1}"
+trap 'docker image rm --force "$image" >/dev/null 2>&1 || true' EXIT
+
+scenarios="$(sed -e 's/#.*//' -e '/^[[:space:]]*$/d' "$root/ci/display-scenarios" | tr '\n' ' ')"
+[[ -n "${scenarios// /}" ]] || {
+  echo "integration: ci/display-scenarios lists no scenario." >&2
+  exit 1
+}
+
+DOCKER_BUILDKIT=1 docker build --quiet --file "$workspace/deploy/Dockerfile" --tag "$image" \
+  "$workspace" >/dev/null
+echo "integration: built the deployment image with Signage; scenarios: ${scenarios}"
+
+cd "$workspace"
+EKKLESIA_SMOKE_TOPOLOGY=split \
+  EKKLESIA_SMOKE_IMAGE="$image" \
+  EKKLESIA_SMOKE_DISPLAY_DEVICE=1 \
+  EKKLESIA_SMOKE_DISPLAY_SCENARIOS="$scenarios" \
+  EKKLESIA_SMOKE_WATCH_MS=120000 \
+  EKKLESIA_SMOKE_RECYCLE_WATCH_MS=120000 \
+  bash scripts/smoke-deploy.sh

ci/platform.env

@@ -0,0 +1,6 @@
+# The Ekklesia platform Signage's CI builds and tests against: one reviewed commit on the platform's
+# `main`, never a moving ref. ci/assemble.sh refuses a commit that is not on `main`.
+#
+# Move it deliberately: change the commit, run `bash ci/update-lock.sh`, and commit both.
+EKKLESIA_PLATFORM_REPOSITORY=https://git.riverofglorymn.org/River_of_Glory_Church/Ekklesia.git
+EKKLESIA_PLATFORM_COMMIT=f50e8f42018c15698709739e2ba54771e97df08a

ci/update-lock.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+# Regenerates ci/pnpm-lock.yaml: the lockfile of Signage assembled into the pinned platform. Run it
+# after changing Signage's dependencies or ci/platform.env, and commit the result. CI installs with
+# --frozen-lockfile, so a stale lockfile fails there rather than resolving anything new.
+#
+#   bash ci/update-lock.sh
+#
+# Uses the platform's own pinned pnpm (its packageManager field) through Corepack.
+set -Eeuo pipefail
+
+root="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")/.." && pwd)"
+work="$(mktemp -d)"
+trap 'rm -rf "$work"' EXIT
+
+bash "$root/ci/assemble.sh" --allow-missing-lock "$work/workspace"
+cd "$work/workspace"
+corepack enable >/dev/null 2>&1 || true
+corepack install >/dev/null
+pnpm install --lockfile-only
+cp pnpm-lock.yaml "$root/ci/pnpm-lock.yaml"
+echo "update-lock: wrote ci/pnpm-lock.yaml"

src/browser/backoff.ts

@@ -0,0 +1,21 @@
+/**
+ * How long a display waits before trying a failed request again.
+ *
+ * Exponential from one second to a one-minute ceiling, so a screen whose server is restarting is
+ * back within seconds, and one whose server is down for an evening asks once a minute rather than
+ * hammering it. The wait is jittered between half and all of that step, so a building's worth of
+ * screens that lost the server at the same moment do not all come back at the same moment.
+ *
+ * There is no attempt limit. A television on a wall has nobody to press Retry, so giving up is the
+ * one thing it must never do.
+ */
+export const DISPLAY_RETRY_BASE_MS = 1_000;
+export const DISPLAY_RETRY_MAX_MS = 60_000;
+
+export function displayRetryDelayMs(attempt: number, random: () => number = Math.random): number {
+  const step = Math.min(
+    DISPLAY_RETRY_MAX_MS,
+    DISPLAY_RETRY_BASE_MS * 2 ** Math.min(Math.max(0, attempt), 16),
+  );
+  return Math.round(step / 2 + (step / 2) * Math.min(1, Math.max(0, random())));
+}

src/display/controller.ts

@@ -1,4 +1,5 @@
 import type { ModuleUiHostContext } from "@ekklesia/module-ui-sdk";
+import { displayRetryDelayMs } from "../browser/backoff.js";
 import {
   type OfflineSnapshot,
   resolveOffline,
@@ -18,6 +19,13 @@ import {
   type SignageMediaCache,
   signageMediaKey,
 } from "../browser/storage.js";
+import {
+  type DisplayChannelState,
+  type DisplayHeartbeat,
+  type DisplayRealtimeGroup,
+  parseViewerSessionResponse,
+  type ViewerSessionResponse,
+} from "../shared/display-protocol.js";
 
 /**
  * The display: what it fetches, what it shows, and what it reports.
@@ -38,36 +46,11 @@ const SNAPSHOT_STORAGE = "last-known-good";
  * must not sit on a stale frame until the next; and a delivery URL has a finite life. */
 const REFRESH_INTERVAL_MS = 4 * 60 * 1000;
 
-export interface DisplayChannelStateView {
+/** The channel state a display renders, exactly as the server sends it (`src/shared`). */
-  readonly serverNowMs: number;
+export type DisplayChannelStateView = DisplayChannelState;
-  readonly channel: { readonly id: string; readonly key: string; readonly groupId: string };
-  readonly crossfadeMs: number;
-  readonly playback: {
-    readonly revision: number;
-    readonly transport: string;
-    readonly blackout: boolean;
-    readonly remainingMs: number | null;
-    readonly selectionRevision: number | null;
-    readonly currentSelectionId: string | null;
-    readonly currentSlotId: string | null;
-    readonly activeTimelineId: string | null;
-    readonly pendingTimelineId: string | null;
-    readonly startedAtMs: number | null;
-    readonly deadlineAtMs: number | null;
-  };
-  readonly timeline: OfflineSnapshot["timeline"];
-  readonly current: SceneSelection | null;
-  readonly staticHold: SceneSelection | null;
-  readonly selections: readonly SceneSelection[];
-}
 
-export interface DisplaySessionView {
+/** The display's registration, exactly as the server answers it (`src/shared`). */
-  readonly viewerId: string;
+export type DisplaySessionView = ViewerSessionResponse;
-  readonly channelId: string;
-  readonly channelKey: string;
-  readonly groupId: string;
-  readonly heartbeatIntervalMs: number;
-}
 
 /** What an operator can see about this screen, and what a test asserts on. Sanitized by
  * construction: identity, counts, and a code. */
@@ -140,6 +123,13 @@ export function createDisplayController(options: DisplayControllerOptions): Disp
   let snapshot: OfflineSnapshot | undefined;
   let offlineActive = false;
   let offlineHandle: unknown;
+  /** Retries, each on its own backoff. A display has nobody to press Retry, so none of them give up;
+   * each resets once its request works again. */
+  let sessionHandle: unknown;
+  let sessionAttempt = 0;
+  let refreshAttempt = 0;
+  let realtimeHandle: unknown;
+  let realtimeAttempt = 0;
   /** Selections this display has tried and failed to render. A broken item is skipped rather than
    * retried in a tight loop, which is what keeps one bad slide from becoming a flicker. */
   const unusable = new Set<string>();
@@ -515,17 +505,119 @@ export function createDisplayController(options: DisplayControllerOptions): Disp
         `channels/${session.channelId}/display-state`,
       );
       if (!response.ok) {
-        online = false;
+        await refreshFailed();
-        await runOffline();
         return;
       }
-      await apply((await response.json()) as DisplayChannelStateView);
+      const state = (await response.json()) as DisplayChannelStateView;
+      refreshAttempt = 0;
+      await apply(state);
     } catch {
-      // The server is unreachable. The frame stays and local continuation takes over; nothing about
+      await refreshFailed();
-      // a network failure belongs on a lobby wall.
+    }
+  }
+
+  /**
+   * The server is unreachable, or answered with an error. The frame stays and local continuation
+   * takes over — nothing about a network failure belongs on a lobby wall — and the refresh is tried
+   * again on its own, because a realtime message that would otherwise prompt it may never come.
+   */
+  async function refreshFailed(): Promise<void> {
     online = false;
     await runOffline();
+    if (stopped) {
+      return;
     }
+    if (refreshHandle !== undefined) {
+      clearTimer(refreshHandle);
+    }
+    refreshHandle = setTimer(() => {
+      refreshHandle = undefined;
+      void refresh();
+    }, displayRetryDelayMs(refreshAttempt));
+    refreshAttempt += 1;
+  }
+
+  /**
+   * Registers this display, and keeps trying until it can.
+   *
+   * Until it has registered, a display continues from its own persisted state, honestly marked as
+   * offline. The first registration that works brings it live — realtime, heartbeats, and fresh
+   * state — exactly as if it had worked the first time.
+   */
+  async function register(): Promise<void> {
+    if (stopped || session) {
+      return;
+    }
+    let registered: DisplaySessionView | undefined;
+    try {
+      const response = await options.context.api.request("viewers/session", {
+        body: JSON.stringify({ viewerKey: await viewerKey() }),
+        headers: { "content-type": "application/json" },
+        method: "POST",
+      });
+      if (response.ok) {
+        // Read, not cast: a registration the display cannot understand is no registration, and is
+        // tried again like any other failure.
+        const parsed = parseViewerSessionResponse(await response.json());
+        registered = parsed.ok ? parsed.value : undefined;
+      }
+    } catch {
+      registered = undefined;
+    }
+    if (stopped) {
+      return;
+    }
+    if (!registered) {
+      online = false;
+      report();
+      sessionHandle = setTimer(() => {
+        sessionHandle = undefined;
+        void register();
+      }, displayRetryDelayMs(sessionAttempt));
+      sessionAttempt += 1;
+      return;
+    }
+    sessionAttempt = 0;
+    session = registered;
+    online = true;
+    subscribe();
+    heartbeatHandle = setInterval(
+      () => void heartbeat(),
+      registered.heartbeatIntervalMs,
+    ) as unknown;
+    await refresh();
+    void heartbeat();
+    report();
+  }
+
+  /**
+   * Subscribes to the display's realtime channel, and again whenever the host reports that a
+   * subscription has ended for good. The host retries ordinary disconnects itself; a stream that
+   * is refused outright ends, and until this resubscribes the display would only learn of changes
+   * at its next slot deadline.
+   */
+  function subscribe(): void {
+    if (stopped || !session) {
+      return;
+    }
+    subscription?.close();
+    subscription = options.context.realtime.subscribe("display-state", {
+      onSnapshot: (state) => onRealtime(state),
+      onStatus: (status) => {
+        online = status === "connected";
+        if (status === "connected") {
+          realtimeAttempt = 0;
+        } else if (status === "closed" && !stopped && realtimeHandle === undefined) {
+          realtimeHandle = setTimer(() => {
+            realtimeHandle = undefined;
+            subscribe();
+          }, displayRetryDelayMs(realtimeAttempt));
+          realtimeAttempt += 1;
+        }
+        report();
+      },
+      onUpdate: (state) => onRealtime(state),
+    });
   }
 
   async function heartbeat(): Promise<void> {
@@ -533,11 +625,10 @@ export function createDisplayController(options: DisplayControllerOptions): Disp
       return;
     }
     try {
-      await options.context.api.request(`viewers/${session.viewerId}/heartbeat`, {
+      // Built as the shared message type, which is the same type the route parses with.
-        body: JSON.stringify({
+      const report: DisplayHeartbeat = {
         cachedAssetCount,
-          error:
+        error: lastError === undefined ? null : { code: lastError.code, detail: lastError.detail },
-            lastError === undefined ? null : { code: lastError.code, detail: lastError.detail },
         offlineReady: snapshot !== undefined && snapshot.selections.length > 0,
         reportedContributionId:
           lastState?.current?.renderer.kind === "contribution"
@@ -547,7 +638,9 @@ export function createDisplayController(options: DisplayControllerOptions): Disp
         reportedProviderId: lastState?.current?.providerId ?? null,
         reportedSelectionId: renderedSelectionId ?? null,
         reportedTimelineId: lastState?.playback.activeTimelineId ?? null,
-        }),
+      };
+      await options.context.api.request(`viewers/${session.viewerId}/heartbeat`, {
+        body: JSON.stringify(report),
         headers: { "content-type": "application/json" },
         method: "POST",
       });
@@ -595,36 +688,7 @@ export function createDisplayController(options: DisplayControllerOptions): Disp
       }
 
       if (telemetry) {
-        try {
+        await register();
-          const response = await options.context.api.request("viewers/session", {
-            body: JSON.stringify({ viewerKey: await viewerKey() }),
-            headers: { "content-type": "application/json" },
-            method: "POST",
-          });
-          if (response.ok) {
-            session = (await response.json()) as DisplaySessionView;
-            online = true;
-          }
-        } catch {
-          online = false;
-        }
-      }
-
-      if (session) {
-        subscription = options.context.realtime.subscribe("display-state", {
-          onSnapshot: (state) => onRealtime(state),
-          onStatus: (status) => {
-            online = status === "connected";
-            report();
-          },
-          onUpdate: (state) => onRealtime(state),
-        });
-        heartbeatHandle = setInterval(
-          () => void heartbeat(),
-          session.heartbeatIntervalMs,
-        ) as unknown;
-        await refresh();
-        void heartbeat();
       }
       report();
     },
@@ -641,6 +705,13 @@ export function createDisplayController(options: DisplayControllerOptions): Disp
         clearTimer(refreshHandle);
         refreshHandle = undefined;
       }
+      for (const handle of [sessionHandle, realtimeHandle]) {
+        if (handle !== undefined) {
+          clearTimer(handle);
+        }
+      }
+      sessionHandle = undefined;
+      realtimeHandle = undefined;
       exitOffline();
       await stage?.destroy();
       stage = undefined;
@@ -658,7 +729,10 @@ export function createDisplayController(options: DisplayControllerOptions): Disp
     if (stopped || !session) {
       return;
     }
-    const groups = (payload as { groups?: readonly Record<string, unknown>[] } | null)?.groups;
+    // Checked at run time as well as typed: realtime state is the platform's transport, and a frame
+    // this display cannot read is simply not one it acts on.
+    const groups = (payload as { groups?: readonly Partial<DisplayRealtimeGroup>[] } | null)
+      ?.groups;
     if (!Array.isArray(groups)) {
       return;
     }

src/server/display/state.ts

@@ -1,4 +1,9 @@
 import type { ModuleStorage } from "@ekklesia/module-sdk";
+import type {
+  DisplayChannelState as DisplayChannelStateWire,
+  DisplaySelection,
+  DisplayTimeline,
+} from "../../shared/display-protocol.js";
 import {
   resolveNativeScene,
   SIGNAGE_NATIVE_PROVIDER_ID,
@@ -35,78 +40,21 @@ import type {
  * and a display refreshes state long before one lapses.
  */
 
-export interface DisplayMedia {
+export type {
-  readonly url: string;
+  DisplayMedia,
-  readonly expiresAtMs: number;
+  DisplaySelection,
-  readonly width: number;
+  DisplayTimeline,
-  readonly height: number;
+  DisplayTimelineSlot,
-}
+} from "../../shared/display-protocol.js";
 
-export interface DisplaySelection {
+/**
-  readonly id: string;
+ * The channel state the server builds: the wire shape a display reads (`src/shared`), plus the
-  readonly revision: number;
+ * channel's name and the group's whole playback snapshot. Declared as an extension so that a field
-  readonly slotId: string | null;
+ * the display relies on cannot be renamed or retyped here without failing to compile.
-  readonly providerId: string;
+ */
-  readonly itemId: string;
+export interface DisplayChannelState extends DisplayChannelStateWire {
-  readonly itemRevision: string;
+  readonly channel: DisplayChannelStateWire["channel"] & { readonly name: string };
-  readonly startedAtMs: number | null;
-  readonly deadlineAtMs: number | null;
-  readonly durationMs: number;
-  readonly availableUntilMs: number | null;
-  readonly withdrawn: boolean;
-  readonly stale: boolean;
-  /** `native-image` is drawn by Signage's own display code; `contribution` is mounted through the
-   * platform's contribution ABI by id. A display never receives a URL for either. */
-  readonly renderer:
-    | { readonly kind: "native-image" }
-    | {
-        readonly kind: "contribution";
-        readonly contributionId: string;
-        readonly contributionVersion: string;
-      }
-    | { readonly kind: "unavailable" };
-  /** The provider's own payload for this channel's audience, or its default. */
-  readonly payload: unknown;
-  readonly presentation: unknown;
-  /** Native images only: fresh delivery URLs, and the already-resolved scene. */
-  readonly media?: { readonly display: DisplayMedia; readonly thumbnail: DisplayMedia };
-  readonly scene?: ReturnType<typeof resolveNativeScene>;
-}
-
-export interface DisplayTimelineSlot {
-  readonly id: string;
-  readonly position: number;
-  readonly enabled: boolean;
-  readonly eligibleFromMs: number | null;
-  readonly eligibleUntilMs: number | null;
-  readonly estimatedDurationMs: number;
-}
-
-export interface DisplayTimeline {
-  readonly id: string;
-  readonly revision: number;
-  readonly eligibleFromMs: number | null;
-  readonly eligibleUntilMs: number | null;
-  readonly slots: readonly DisplayTimelineSlot[];
-}
-
-export interface DisplayChannelState {
-  readonly serverNowMs: number;
-  readonly channel: {
-    readonly id: string;
-    readonly key: string;
-    readonly name: string;
-    readonly groupId: string;
-  };
-  readonly crossfadeMs: number;
   readonly playback: PlaybackSnapshot;
-  readonly timeline: DisplayTimeline | null;
-  readonly current: DisplaySelection | null;
-  /** The held image, already resolved as a scene. Separate from `current` because a static hold sits
-   * *over* a preserved cursor rather than replacing it. */
-  readonly staticHold: DisplaySelection | null;
-  /** Bounded, newest first. Material for offline continuation, never a substitute for authority. */
-  readonly selections: readonly DisplaySelection[];
 }
 
 /** How many resolved selections travel to a display. Enough to cover a pattern and then some, small

src/server/routes/viewers.ts

@@ -1,13 +1,14 @@
 import type { ModuleHttpRouteRegistrar, ModuleRouteHandlerContext } from "@ekklesia/module-sdk";
+import {
+  type DisplayHeartbeatResponse,
+  type ProtocolResult,
+  parseDisplayHeartbeat,
+  parseViewerSessionRequest,
+  type ViewerSessionResponse,
+} from "../../shared/display-protocol.js";
 import { buildDisplayChannelState } from "../display/state.js";
 import { SignageError, signageBoundary } from "../errors.js";
-import {
+import { optionalString, readJsonObject, requireUuidParam } from "../http.js";
-  optionalBoolean,
-  optionalInteger,
-  optionalString,
-  readJsonObject,
-  requireUuidParam,
-} from "../http.js";
 import type { SignagePlaybackService } from "../playback/service.js";
 import type { SignageProviderRegistry } from "../providers/registry.js";
 import { readSignageSettings } from "../settings.js";
@@ -21,12 +22,24 @@ import {
   updateViewerAssignment,
 } from "../viewers/service.js";
 
-const OPAQUE_ID = /^[\w][\w.:-]{0,199}$/u;
+/** How a route reaches Signage's tables. Production uses the module database; the display contract
+ * test supplies the in-memory store so the real handlers can answer the real display controller. */
+/** How often a display checks in. Sent to the display rather than built into it, so the server can
+ * change its own expectations without a display deployment. */
+export const SIGNAGE_HEARTBEAT_INTERVAL_MS = 12_000;
 
-function store(context: ModuleRouteHandlerContext): SignageStore {
+/** A display message the shared protocol refused, as Signage's own sanitized code naming the field. */
-  return createSignageStore(context.services.database);
+function accepted<Value>(result: ProtocolResult<Value>): Value {
+  if (!result.ok) {
+    throw new SignageError("SIGNAGE_INVALID_REQUEST", { field: result.field });
+  }
+  return result.value;
 }
 
+export type ViewerRouteStore = (context: ModuleRouteHandlerContext) => SignageStore;
+
+const databaseStore: ViewerRouteStore = (context) => createSignageStore(context.services.database);
+
 /**
  * The safe Core device identifier, when the caller is a paired display.
  *
@@ -57,6 +70,7 @@ export function registerViewerRoutes(
   http: ModuleHttpRouteRegistrar,
   playback: () => SignagePlaybackService,
   registry: () => SignageProviderRegistry,
+  store: ViewerRouteStore = databaseStore,
 ): void {
   http.route({
     method: "POST",
@@ -65,11 +79,7 @@ export function registerViewerRoutes(
     maxBodyBytes: 2048,
     handler(request, context) {
       return signageBoundary(async () => {
-        const body = await readJsonObject(request);
+        const { viewerKey } = accepted(parseViewerSessionRequest(await readJsonObject(request)));
-        const viewerKey = body.viewerKey;
-        if (typeof viewerKey !== "string") {
-          throw new SignageError("SIGNAGE_INVALID_REQUEST", { field: "viewerKey" });
-        }
         // A display registers with the identity it keeps in its own IndexedDB and lands on the
         // default channel. It never chooses one: a public screen that could would let anyone who
         // reaches the page decide what a wall shows (ADR 0004).
@@ -79,14 +89,15 @@ export function registerViewerRoutes(
           store: store(context),
           viewerKey,
         });
-        return Response.json({
+        const response: ViewerSessionResponse = {
           channelId: session.channel.id,
           channelKey: session.channel.key,
           groupId: session.groupId,
-          heartbeatIntervalMs: 12_000,
+          heartbeatIntervalMs: SIGNAGE_HEARTBEAT_INTERVAL_MS,
           viewerId: session.viewer.id,
           viewerName: session.viewer.name,
-        });
+        };
+        return Response.json(response);
       });
     },
   });
@@ -99,52 +110,21 @@ export function registerViewerRoutes(
     handler(request, context) {
       return signageBoundary(async () => {
         const viewerId = requireUuidParam(context, "viewerId");
-        const body = await readJsonObject(request);
+        const heartbeat = accepted(parseDisplayHeartbeat(await readJsonObject(request)));
         const settings = readSignageSettings(context.config);
-        const rawError = body.error;
         // Telemetry only. Nothing a display reports here changes authoritative playback, and a
         // local cursor is never accepted as truth.
         const result = await recordViewerHeartbeat({
           at: new Date(),
-          heartbeat: {
+          heartbeat,
-            cachedAssetCount:
-              optionalInteger(body, "cachedAssetCount", {
-                maximum: 100_000,
-                minimum: 0,
-              }) ?? undefined,
-            offlineReady: optionalBoolean(body, "offlineReady") ?? undefined,
-            reportedContributionId:
-              optionalString(body, "reportedContributionId", { maxLength: 200 }) ?? null,
-            reportedPlaybackRevision:
-              optionalInteger(body, "reportedPlaybackRevision", {
-                maximum: Number.MAX_SAFE_INTEGER,
-                minimum: 0,
-              }) ?? null,
-            reportedProviderId:
-              optionalString(body, "reportedProviderId", { maxLength: 200 }) ?? null,
-            reportedSelectionId:
-              optionalString(body, "reportedSelectionId", { maxLength: 64 }) ?? null,
-            reportedTimelineId:
-              optionalString(body, "reportedTimelineId", { maxLength: 64 }) ?? null,
-            ...(rawError === undefined
-              ? {}
-              : {
-                  error:
-                    rawError === null
-                      ? null
-                      : {
-                          code: readErrorCode(rawError),
-                          detail: readErrorDetail(rawError),
-                        },
-                }),
-          },
           minimumWriteIntervalMs: settings.viewerHeartbeatWriteMs,
           store: store(context),
           viewerId,
         });
         // The response says what a display needs to keep itself honest, and nothing about other
         // displays or the installation.
-        return Response.json({ acknowledged: true, recorded: result.written });
+        const response: DisplayHeartbeatResponse = { acknowledged: true, recorded: result.written };
+        return Response.json(response);
       });
     },
   });
@@ -235,25 +215,3 @@ export function registerViewerRoutes(
     },
   });
 }
-
-function readErrorCode(value: unknown): string {
-  if (typeof value !== "object" || value === null) {
-    throw new SignageError("SIGNAGE_INVALID_REQUEST", { field: "error" });
-  }
-  const code = (value as { code?: unknown }).code;
-  if (typeof code !== "string" || !OPAQUE_ID.test(code)) {
-    throw new SignageError("SIGNAGE_INVALID_REQUEST", { field: "error.code" });
-  }
-  return code;
-}
-
-function readErrorDetail(value: unknown): string | null {
-  const detail = (value as { detail?: unknown }).detail;
-  if (detail === undefined || detail === null) {
-    return null;
-  }
-  if (typeof detail !== "string") {
-    throw new SignageError("SIGNAGE_INVALID_REQUEST", { field: "error.detail" });
-  }
-  return detail;
-}

src/server/runtime.ts

@@ -3,6 +3,7 @@ import {
   type ModuleRuntimeContext,
   type ModuleRuntimeDefinition,
 } from "@ekklesia/module-sdk";
+import type { DisplayRealtimePayload } from "../shared/display-protocol.js";
 import { SIGNAGE_SLIDE_RENDERER_INPUT_VERSION } from "../shared/renderer-input.js";
 import {
   SIGNAGE_IMAGE_MAX_BYTES,
@@ -98,10 +99,10 @@ export function createSignageModuleRuntime(
       left.groupId.localeCompare(right.groupId),
     );
     try {
-      context.realtime.publish(SIGNAGE_DISPLAY_CHANNEL, {
+      // The shape the display reads (`src/shared/display-protocol.ts`): each group's whole snapshot
-        groups,
+      // carries the revisions it compares.
-        serverNowMs: now().getTime(),
+      const display: DisplayRealtimePayload = { groups, serverNowMs: now().getTime() };
-      });
+      context.realtime.publish(SIGNAGE_DISPLAY_CHANNEL, display);
       context.realtime.publish(SIGNAGE_MANAGEMENT_CHANNEL, {
         // Revisions only. The dashboard refetches what changed over authorized HTTP.
         groups: groups.map((snapshot) => ({

src/shared/display-protocol.ts

@@ -0,0 +1,318 @@
+/**
+ * What a display and Signage's server say to each other, defined once.
+ *
+ * The display controller builds these messages and the viewer routes read them, from this one file.
+ * They used to be described twice — once in the controller's own interfaces and once in each
+ * route's field-by-field parsing — and the two descriptions disagreed about one thing: the display
+ * sent `null` for "nothing to report" and the heartbeat route accepted only a *missing* field. Every
+ * heartbeat from a screen showing a native image was refused with a 400, silently, for as long as it
+ * ran. Nothing here can drift that way again, because there is nothing to drift between.
+ *
+ * Three rules hold for every message:
+ *
+ * - **A nullable field accepts `null` explicitly.** `null` means "I have nothing", and is a value.
+ * - **An absent field means "not reported".** A receiver keeps whatever it already had.
+ * - **An unknown field is ignored, never refused.** A newer display reporting something an older
+ *   server does not know yet must still be heard; refusing it would be this same bug in reverse.
+ *
+ * Like the rest of `src/shared`, it imports nothing from outside `src/shared`: it is compiled into the browser
+ * artifact as well as the server, and must pull neither the module SDK nor Node into a display.
+ */
+
+import type { SignageNativeScene } from "./native-images.js";
+
+// --- Parsing ---------------------------------------------------------------------------------------
+
+/** The outcome of reading one message: the value, or the one field that was wrong. Never a parser's
+ * own message, which is not something a client or an operator should see. */
+export type ProtocolResult<Value> =
+  | { readonly ok: true; readonly value: Value }
+  | { readonly ok: false; readonly field: string };
+
+type Reader<Value> = (raw: unknown) => { readonly ok: true; readonly value: Value } | undefined;
+
+function isRecord(value: unknown): value is Record<string, unknown> {
+  return typeof value === "object" && value !== null && !Array.isArray(value);
+}
+
+function text(options: { readonly maxLength: number; readonly pattern?: RegExp }): Reader<string> {
+  return (raw) => {
+    if (typeof raw !== "string") {
+      return undefined;
+    }
+    const value = raw.trim();
+    if (value.length === 0 || value.length > options.maxLength) {
+      return undefined;
+    }
+    if (options.pattern && !options.pattern.test(value)) {
+      return undefined;
+    }
+    return { ok: true, value };
+  };
+}
+
+function integer(options: { readonly minimum: number; readonly maximum: number }): Reader<number> {
+  return (raw) =>
+    typeof raw === "number" &&
+    Number.isSafeInteger(raw) &&
+    raw >= options.minimum &&
+    raw <= options.maximum
+      ? { ok: true, value: raw }
+      : undefined;
+}
+
+const flag: Reader<boolean> = (raw) =>
+  typeof raw === "boolean" ? { ok: true, value: raw } : undefined;
+
+function nullable<Value>(read: Reader<Value>): Reader<Value | null> {
+  return (raw) => (raw === null ? { ok: true, value: null } : read(raw));
+}
+
+/** Reads the named fields of a message. Absent fields are left out of the result; any other field
+ * of the object is ignored. */
+function readFields<Shape extends object>(
+  raw: unknown,
+  readers: { readonly [Key in keyof Shape]-?: Reader<Exclude<Shape[Key], undefined>> },
+): ProtocolResult<Shape> {
+  if (!isRecord(raw)) {
+    return { field: "body", ok: false };
+  }
+  const value: Record<string, unknown> = {};
+  for (const key of Object.keys(readers) as (keyof Shape & string)[]) {
+    if (raw[key] === undefined) {
+      continue;
+    }
+    const read = readers[key](raw[key]);
+    if (!read) {
+      return { field: key, ok: false };
+    }
+    value[key] = read.value;
+  }
+  return { ok: true, value: value as Shape };
+}
+
+function requireFields<Shape>(
+  result: ProtocolResult<Partial<Shape>>,
+  required: readonly (keyof Shape & string)[],
+): ProtocolResult<Shape> {
+  if (!result.ok) {
+    return result;
+  }
+  for (const key of required) {
+    if (result.value[key] === undefined) {
+      return { field: key, ok: false };
+    }
+  }
+  return result as ProtocolResult<Shape>;
+}
+
+// --- Limits ----------------------------------------------------------------------------------------
+
+/** A display's own identity, kept in its IndexedDB. */
+export const DISPLAY_VIEWER_KEY = /^[\w-]{8,128}$/u;
+/** The longest error detail a display may report. Operator-facing text a browser supplied, so it is
+ * bounded, and because an unbounded one is a way to fill a table. */
+export const DISPLAY_ERROR_DETAIL_MAX = 200;
+export const DISPLAY_ERROR_CODE = /^[\w][\w.:-]{0,63}$/u;
+export const DISPLAY_CACHED_ASSET_MAX = 100_000;
+
+// --- Viewer session: POST viewers/session ----------------------------------------------------------
+
+export interface ViewerSessionRequest {
+  readonly viewerKey: string;
+}
+
+export interface ViewerSessionResponse {
+  readonly viewerId: string;
+  readonly viewerName: string;
+  readonly channelId: string;
+  readonly channelKey: string;
+  readonly groupId: string;
+  readonly heartbeatIntervalMs: number;
+}
+
+const identifier = text({ maxLength: 200 });
+
+export function parseViewerSessionRequest(raw: unknown): ProtocolResult<ViewerSessionRequest> {
+  return requireFields<ViewerSessionRequest>(
+    readFields<Partial<ViewerSessionRequest>>(raw, {
+      viewerKey: text({ maxLength: 128, pattern: DISPLAY_VIEWER_KEY }),
+    }),
+    ["viewerKey"],
+  );
+}
+
+export function parseViewerSessionResponse(raw: unknown): ProtocolResult<ViewerSessionResponse> {
+  return requireFields<ViewerSessionResponse>(
+    readFields<Partial<ViewerSessionResponse>>(raw, {
+      channelId: identifier,
+      channelKey: identifier,
+      groupId: identifier,
+      heartbeatIntervalMs: integer({ maximum: 600_000, minimum: 1_000 }),
+      viewerId: identifier,
+      viewerName: text({ maxLength: 200 }),
+    }),
+    ["viewerId", "viewerName", "channelId", "channelKey", "groupId", "heartbeatIntervalMs"],
+  );
+}
+
+// --- Heartbeat: POST viewers/:viewerId/heartbeat ---------------------------------------------------
+
+export interface DisplayReportedError {
+  readonly code: string;
+  readonly detail: string | null;
+}
+
+/**
+ * What a display reports about itself. Telemetry, never authority: nothing here changes playback.
+ *
+ * Every field is optional on the wire (absent = not reported). The display sends all of them.
+ */
+export interface DisplayHeartbeat {
+  readonly cachedAssetCount?: number;
+  readonly offlineReady?: boolean;
+  readonly reportedTimelineId?: string | null;
+  readonly reportedSelectionId?: string | null;
+  readonly reportedProviderId?: string | null;
+  readonly reportedContributionId?: string | null;
+  readonly reportedPlaybackRevision?: number | null;
+  readonly error?: DisplayReportedError | null;
+}
+
+const reportedError: Reader<DisplayReportedError> = (raw) => {
+  const read = readFields<{ code?: string; detail?: string | null }>(raw, {
+    code: text({ maxLength: 64, pattern: DISPLAY_ERROR_CODE }),
+    detail: nullable(text({ maxLength: DISPLAY_ERROR_DETAIL_MAX })),
+  });
+  if (!read.ok || read.value.code === undefined) {
+    return undefined;
+  }
+  return { ok: true, value: { code: read.value.code, detail: read.value.detail ?? null } };
+};
+
+export function parseDisplayHeartbeat(raw: unknown): ProtocolResult<DisplayHeartbeat> {
+  return readFields<DisplayHeartbeat>(raw, {
+    cachedAssetCount: integer({ maximum: DISPLAY_CACHED_ASSET_MAX, minimum: 0 }),
+    error: nullable(reportedError),
+    offlineReady: flag,
+    reportedContributionId: nullable(text({ maxLength: 200 })),
+    reportedPlaybackRevision: nullable(integer({ maximum: Number.MAX_SAFE_INTEGER, minimum: 0 })),
+    reportedProviderId: nullable(text({ maxLength: 200 })),
+    reportedSelectionId: nullable(text({ maxLength: 64 })),
+    reportedTimelineId: nullable(text({ maxLength: 64 })),
+  });
+}
+
+export interface DisplayHeartbeatResponse {
+  readonly acknowledged: true;
+  readonly recorded: boolean;
+}
+
+// --- Channel state: GET channels/:channelId/display-state -----------------------------------------
+
+/** A fresh, signed delivery URL. A bearer capability with an expiry: never persisted by a display. */
+export interface DisplayMedia {
+  readonly url: string;
+  readonly expiresAtMs: number;
+  readonly width: number;
+  readonly height: number;
+}
+
+export type DisplayRenderer =
+  | { readonly kind: "native-image" }
+  | {
+      readonly kind: "contribution";
+      readonly contributionId: string;
+      readonly contributionVersion: string;
+    }
+  | { readonly kind: "unavailable" };
+
+export interface DisplaySelection {
+  readonly id: string;
+  readonly revision: number;
+  readonly slotId: string | null;
+  readonly providerId: string;
+  readonly itemId: string;
+  readonly itemRevision: string;
+  readonly startedAtMs: number | null;
+  readonly deadlineAtMs: number | null;
+  readonly durationMs: number;
+  readonly availableUntilMs: number | null;
+  readonly withdrawn: boolean;
+  readonly stale: boolean;
+  /** `native-image` is drawn by Signage's own display code; `contribution` is mounted through the
+   * platform's contribution ABI by id. A display never receives a URL for either. */
+  readonly renderer: DisplayRenderer;
+  /** The provider's own payload for this channel's audience, or its default. */
+  readonly payload: unknown;
+  readonly presentation: unknown;
+  /** Native images only: fresh delivery URLs, and the already-resolved scene. */
+  readonly media?: { readonly display: DisplayMedia; readonly thumbnail: DisplayMedia };
+  readonly scene?: SignageNativeScene;
+}
+
+export interface DisplayTimelineSlot {
+  readonly id: string;
+  readonly position: number;
+  readonly enabled: boolean;
+  readonly eligibleFromMs: number | null;
+  readonly eligibleUntilMs: number | null;
+  readonly estimatedDurationMs: number;
+}
+
+export interface DisplayTimeline {
+  readonly id: string;
+  readonly revision: number;
+  readonly eligibleFromMs: number | null;
+  readonly eligibleUntilMs: number | null;
+  readonly slots: readonly DisplayTimelineSlot[];
+}
+
+/** The part of the group's playback a display acts on. The server sends its whole snapshot; this is
+ * what a display may rely on being there. */
+export interface DisplayPlayback {
+  readonly revision: number;
+  readonly transport: string;
+  readonly blackout: boolean;
+  readonly remainingMs: number | null;
+  readonly selectionRevision: number | null;
+  readonly currentSelectionId: string | null;
+  readonly currentSlotId: string | null;
+  readonly activeTimelineId: string | null;
+  readonly pendingTimelineId: string | null;
+  readonly startedAtMs: number | null;
+  readonly deadlineAtMs: number | null;
+}
+
+export interface DisplayChannelState {
+  readonly serverNowMs: number;
+  readonly channel: {
+    readonly id: string;
+    readonly key: string;
+    readonly groupId: string;
+  };
+  readonly crossfadeMs: number;
+  readonly playback: DisplayPlayback;
+  readonly timeline: DisplayTimeline | null;
+  readonly current: DisplaySelection | null;
+  /** The held image, already resolved as a scene. Separate from `current` because a static hold sits
+   * *over* a preserved cursor rather than replacing it. */
+  readonly staticHold: DisplaySelection | null;
+  /** Bounded, newest first. Material for offline continuation, never a substitute for authority. */
+  readonly selections: readonly DisplaySelection[];
+}
+
+// --- Realtime: the display-state channel ----------------------------------------------------------
+
+/** One group's revisions on the realtime channel. Compact on purpose: a revision this display has
+ * not applied means "fetch the state you are authorized for", never content. */
+export interface DisplayRealtimeGroup {
+  readonly groupId: string;
+  readonly revision: number;
+  readonly selectionRevision: number | null;
+}
+
+export interface DisplayRealtimePayload {
+  readonly groups: readonly DisplayRealtimeGroup[];
+  readonly serverNowMs: number;
+}

tests/browser/backoff.test.ts

@@ -0,0 +1,34 @@
+import { describe, expect, it } from "vitest";
+import {
+  DISPLAY_RETRY_BASE_MS,
+  DISPLAY_RETRY_MAX_MS,
+  displayRetryDelayMs,
+} from "../../src/browser/backoff.js";
+
+describe("display retry backoff", () => {
+  it("doubles from one second and stops at one minute", () => {
+    const longest = (attempt: number) => displayRetryDelayMs(attempt, () => 1);
+    expect([0, 1, 2, 3, 4, 5, 6, 7, 50].map(longest)).toEqual([
+      1_000, 2_000, 4_000, 8_000, 16_000, 32_000, 60_000, 60_000, 60_000,
+    ]);
+  });
+
+  it("jitters between half and all of each step, and never waits zero", () => {
+    for (const attempt of [0, 3, 6, 40]) {
+      const step = Math.min(DISPLAY_RETRY_MAX_MS, DISPLAY_RETRY_BASE_MS * 2 ** attempt);
+      expect(displayRetryDelayMs(attempt, () => 0)).toBe(step / 2);
+      expect(displayRetryDelayMs(attempt, () => 1)).toBe(step);
+      for (let index = 0; index < 50; index += 1) {
+        const delay = displayRetryDelayMs(attempt);
+        expect(delay).toBeGreaterThanOrEqual(step / 2);
+        expect(delay).toBeLessThanOrEqual(step);
+      }
+    }
+  });
+
+  it("treats a nonsensical attempt or random value as the nearest sensible one", () => {
+    expect(displayRetryDelayMs(-3, () => 1)).toBe(1_000);
+    expect(displayRetryDelayMs(2, () => 7)).toBe(4_000);
+    expect(displayRetryDelayMs(2, () => -1)).toBe(2_000);
+  });
+});

tests/browser/contributed-renderer.test.tsx

@@ -43,6 +43,7 @@ const SELECTION = {
   },
   revision: 1,
   slotId: "slot-trivia",
+  stale: false,
   startedAtMs: 0,
   withdrawn: false,
 };

tests/browser/display-contract.test.ts

@@ -0,0 +1,223 @@
+// @vitest-environment happy-dom
+import type { ModuleRouteDefinitionInput, ModuleRouteHandlerContext } from "@ekklesia/module-sdk";
+import { beforeEach, describe, expect, it } from "vitest";
+import { createMemoryMediaCache, createMemoryStore } from "../../src/browser/storage.js";
+import { createDisplayController } from "../../src/display/controller.js";
+import { ingestNativeImage } from "../../src/server/images/service.js";
+import {
+  createPlaybackService,
+  type SignagePlaybackService,
+} from "../../src/server/playback/service.js";
+import { createNativeImageProvider } from "../../src/server/providers/native-provider.js";
+import {
+  createProviderRegistry,
+  type SignageProviderRegistry,
+} from "../../src/server/providers/registry.js";
+import { registerViewerRoutes } from "../../src/server/routes/viewers.js";
+import { addSlot, createTimeline } from "../../src/server/timelines/service.js";
+import { SIGNAGE_NATIVE_PROVIDER_ID } from "../../src/shared/native-images.js";
+import { SIGNAGE_DEFAULT_SETTINGS } from "../../src/shared/settings.js";
+import { createFakeStorage, type FakeStorage } from "../support/fake-storage.js";
+import { makeImage } from "../support/images.js";
+import { createMemorySignageStore, type MemorySignageStore } from "../support/memory-store.js";
+import { fakeConfiguration, fakeHandlerContext } from "../support/module-harness.js";
+import { createFakeHost, type FakeHost } from "./support/fake-host.js";
+
+/**
+ * The display and the server, speaking to each other for real.
+ *
+ * Every other display test answers the controller with a hand-written stub, and every server test
+ * calls the service with a hand-written object. Each side was therefore only ever tested against its
+ * own idea of the other, and in production every heartbeat a display sent was refused with a 400:
+ * the display sent `null` for an absent value and the route accepted only a missing field.
+ *
+ * Here the **real** display controller's requests are answered by the **real** viewer routes, over
+ * Signage's in-memory store. A request one side builds and the other refuses fails this suite.
+ */
+
+const START = Date.parse("2026-10-05T15:00:00.000Z");
+const DEVICE_ID = "6f1c2d3e-4b5a-4c6d-8e7f-901a2b3c4d5e";
+
+interface RouteCall {
+  readonly method: string;
+  readonly path: string;
+  readonly status: number;
+  readonly code: string | undefined;
+}
+
+/** Matches a relative module path against the routes the module registered, and runs the handler
+ * exactly as the platform would: one Request, one handler context. */
+function routeTable(routes: readonly ModuleRouteDefinitionInput[]) {
+  return routes.map((route) => {
+    const names: string[] = [];
+    const pattern = new RegExp(
+      `^${route.path.replace(/:(\w+)/gu, (_match, name: string) => {
+        names.push(name);
+        return "([^/]+)";
+      })}$`,
+      "u",
+    );
+    return { names, pattern, route };
+  });
+}
+
+describe("display ↔ server contract", () => {
+  let clock: number;
+  let counter: number;
+  let store: MemorySignageStore;
+  let storage: FakeStorage;
+  let registry: SignageProviderRegistry;
+  let playback: SignagePlaybackService;
+  let calls: RouteCall[];
+  let host: FakeHost;
+  let root: HTMLElement;
+
+  const newId = () => {
+    counter += 1;
+    return `00000000-0000-4000-8000-${String(counter).padStart(12, "0")}`;
+  };
+
+  beforeEach(async () => {
+    clock = START;
+    counter = 0;
+    calls = [];
+    store = createMemorySignageStore({ now: () => new Date(clock) });
+    storage = createFakeStorage({ now: () => new Date(clock) });
+    registry = createProviderRegistry({ now: () => new Date(clock), timeoutMs: 1000 });
+    registry.capability.register(
+      createNativeImageProvider({ defaultDurationMs: 7000, now: () => new Date(clock), store })
+        .registration,
+    );
+    playback = createPlaybackService({
+      newId,
+      now: () => clock,
+      registry,
+      setTimer: () => undefined,
+      settings: SIGNAGE_DEFAULT_SETTINGS,
+      store,
+    });
+    await playback.ensureDefaultGroup();
+
+    // One native image on a pinned slot: the most ordinary thing a wall shows, and exactly the case
+    // whose heartbeat carries no contribution id.
+    const image = await ingestNativeImage({
+      bytes: await makeImage({ format: "png", height: 1080, width: 1920 }),
+      displayName: "Welcome",
+      expiresAt: null,
+      filename: "welcome.png",
+      storage,
+      store,
+    });
+    const timeline = await createTimeline({ name: "Lobby", newId, store });
+    await addSlot({
+      newId,
+      pinned: { itemId: image.image.id, providerId: SIGNAGE_NATIVE_PROVIDER_ID },
+      store,
+      timelineId: timeline.timeline.id,
+    });
+    const groups = await store.listChannelGroups();
+    await playback.activate(groups[0]?.group.id as string, timeline.timeline.id, "now");
+
+    const registered: ModuleRouteDefinitionInput[] = [];
+    registerViewerRoutes(
+      { route: (definition) => registered.push(definition) },
+      () => playback,
+      () => registry,
+      () => store,
+    );
+    const table = routeTable(registered);
+
+    host = createFakeHost({
+      permissions: ["signage.display.view"],
+      principal: { deviceId: DEVICE_ID, kind: "device" } as never,
+      routes: [],
+    });
+    Object.assign(host.context.api, {
+      async request(path: string, init?: RequestInit): Promise<Response> {
+        const method = (init?.method ?? "GET").toUpperCase();
+        const relative = `/${path.replace(/^\//u, "").split("?")[0]}`;
+        for (const { names, pattern, route } of table) {
+          const match = pattern.exec(relative);
+          if (route.method !== method || !match) {
+            continue;
+          }
+          const params = Object.fromEntries(names.map((name, index) => [name, match[index + 1]]));
+          const context: ModuleRouteHandlerContext = fakeHandlerContext({
+            config: fakeConfiguration({}),
+            params: Object.freeze(params) as never,
+            principal: { deviceId: DEVICE_ID, kind: "device" } as never,
+            storage,
+          });
+          const request = new Request(
+            `https://api.example.test/api/v1/modules/signage${relative}`,
+            {
+              ...(init?.body === undefined ? {} : { body: init.body }),
+              headers: init?.headers ?? {},
+              method,
+            },
+          );
+          const response = await route.handler(request, context);
+          const body = (await response
+            .clone()
+            .json()
+            .catch(() => null)) as { code?: string } | null;
+          calls.push({ code: body?.code, method, path: route.path, status: response.status });
+          return response;
+        }
+        throw new Error(
+          `The display asked for ${method} ${relative}, which Signage does not serve.`,
+        );
+      },
+    });
+
+    root = document.createElement("div");
+    document.body.append(root);
+  });
+
+  async function settle(): Promise<void> {
+    for (let index = 0; index < 20; index += 1) {
+      await new Promise((resolve) => setTimeout(resolve, 0));
+    }
+  }
+
+  function controller() {
+    return createDisplayController({
+      context: host.context,
+      decodeImage: () => Promise.resolve(),
+      media: createMemoryMediaCache(),
+      newViewerKey: () => "kiosk-contract-0001",
+      now: () => clock,
+      root,
+      store: createMemoryStore(),
+    });
+  }
+
+  it("the server accepts every request the display sends", async () => {
+    const display = controller();
+    await display.start();
+    await settle();
+    await display.stop();
+
+    const sent = calls.map((call) => `${call.method} ${call.path}`);
+    expect(sent).toContain("POST /viewers/session");
+    expect(sent).toContain("GET /channels/:channelId/display-state");
+    expect(sent).toContain("POST /viewers/:viewerId/heartbeat");
+    // Not one refusal, of any kind, for anything the display built itself.
+    expect(calls.filter((call) => call.status >= 400)).toEqual([]);
+  });
+
+  it("a heartbeat from a screen showing a native image is recorded", async () => {
+    const display = controller();
+    await display.start();
+    await settle();
+    await display.stop();
+
+    const [viewer] = await store.listViewers();
+    expect(viewer).toBeDefined();
+    // What the display reported is what the roster now holds — which is only true if the server
+    // took the heartbeat, rather than refusing it and leaving the session's values in place.
+    expect(viewer?.reportedSelectionId).not.toBeNull();
+    expect(viewer?.reportedProviderId).toBe(SIGNAGE_NATIVE_PROVIDER_ID);
+    expect(viewer?.reportedContributionId).toBeNull();
+  });
+});

tests/browser/display-recovery.test.ts

@@ -0,0 +1,307 @@
+// @vitest-environment happy-dom
+import type { ModuleUiHostContext } from "@ekklesia/module-ui-sdk";
+import { beforeEach, describe, expect, it } from "vitest";
+import {
+  createMemoryMediaCache,
+  createMemoryStore,
+  type SignageLocalStore,
+} from "../../src/browser/storage.js";
+import {
+  createDisplayController,
+  type DisplayChannelStateView,
+} from "../../src/display/controller.js";
+import { createFakeHost, type FakeHost, jsonResponse } from "./support/fake-host.js";
+
+/**
+ * A display never reaches a dead end.
+ *
+ * Every network path a display depends on — registering, fetching state, realtime — can fail once,
+ * as a kiosk that boots before its network settles or an API that restarts will make it. Each must
+ * be tried again, with backoff, until it works, and the display must then be exactly as live as if
+ * nothing had happened. None of it may need a person to reload a television.
+ *
+ * Time is virtual: every timer the controller schedules goes through the queue below.
+ */
+
+const SESSION = {
+  channelId: "chan-1",
+  channelKey: "default",
+  groupId: "group-1",
+  heartbeatIntervalMs: 12_000,
+  viewerId: "11111111-1111-4111-8111-111111111111",
+  viewerName: "Foyer",
+};
+
+function selection(id: string) {
+  return {
+    availableUntilMs: null,
+    deadlineAtMs: 5000,
+    durationMs: 5000,
+    id,
+    itemId: `image-${id}`,
+    itemRevision: "rev-1",
+    media: {
+      display: {
+        expiresAtMs: 900_000,
+        height: 1080,
+        url: `https://cdn.invalid/${id}`,
+        width: 1920,
+      },
+      thumbnail: {
+        expiresAtMs: 900_000,
+        height: 270,
+        url: `https://cdn.invalid/${id}-t`,
+        width: 480,
+      },
+    },
+    payload: { kind: "native-image" },
+    presentation: null,
+    providerId: "signage.native-images",
+    renderer: { kind: "native-image" as const },
+    revision: 1,
+    scene: {
+      backgroundColor: "#000000",
+      backgroundImage: false,
+      backgroundMode: "none" as const,
+      blurPixels: 0,
+      darkenOpacity: 0,
+      fit: "fill" as const,
+      shadow: false,
+    },
+    slotId: "slot-1",
+    stale: false,
+    startedAtMs: 0,
+    withdrawn: false,
+  };
+}
+
+function channelState(revision = 3): DisplayChannelStateView {
+  const current = selection("sel-1");
+  return {
+    channel: { groupId: "group-1", id: "chan-1", key: "default" },
+    crossfadeMs: 0,
+    current,
+    playback: {
+      activeTimelineId: "timeline-1",
+      blackout: false,
+      currentSelectionId: current.id,
+      currentSlotId: current.slotId,
+      deadlineAtMs: 5000,
+      pendingTimelineId: null,
+      remainingMs: 5000,
+      revision,
+      selectionRevision: 1,
+      startedAtMs: 0,
+      transport: "playing",
+    },
+    selections: [current],
+    serverNowMs: 0,
+    staticHold: null,
+    timeline: null,
+  };
+}
+
+/** Server behaviour that a test can change mid-run. */
+interface Server {
+  sessionFailures: number;
+  stateFailing: boolean;
+  sessionRequests: number;
+  stateRequests: number;
+}
+
+describe("a display recovers from every failed request without a reload", () => {
+  let clock: number;
+  let timers: Map<number, { at: number; run: () => void }>;
+  let nextTimer: number;
+  let root: HTMLElement;
+  let local: SignageLocalStore;
+  let server: Server;
+  let host: FakeHost;
+  let subscriptions: { onStatus?: (status: string) => void }[];
+
+  beforeEach(() => {
+    clock = 0;
+    timers = new Map();
+    nextTimer = 1;
+    root = document.createElement("div");
+    document.body.append(root);
+    local = createMemoryStore();
+    server = { sessionFailures: 0, sessionRequests: 0, stateFailing: false, stateRequests: 0 };
+    subscriptions = [];
+    host = createFakeHost({
+      permissions: ["signage.display.view"],
+      routes: [
+        {
+          method: "POST",
+          path: "viewers/session",
+          respond: () => {
+            server.sessionRequests += 1;
+            if (server.sessionFailures > 0) {
+              server.sessionFailures -= 1;
+              throw new TypeError("Failed to fetch");
+            }
+            return jsonResponse(SESSION);
+          },
+        },
+        {
+          method: "GET",
+          path: "channels/chan-1/display-state",
+          respond: () => {
+            server.stateRequests += 1;
+            if (server.stateFailing) {
+              throw new TypeError("Failed to fetch");
+            }
+            return jsonResponse(channelState());
+          },
+        },
+        { method: "POST", path: /^viewers\/.*\/heartbeat$/u, respond: () => jsonResponse({}) },
+      ],
+    });
+    Object.assign(host.context, {
+      realtime: {
+        subscribe: (_channel: string, handlers: { onStatus?: (status: string) => void }) => {
+          subscriptions.push(handlers);
+          return { close: () => undefined };
+        },
+      } satisfies Pick<ModuleUiHostContext["realtime"], "subscribe"> as never,
+    });
+  });
+
+  async function settle(): Promise<void> {
+    for (let index = 0; index < 20; index += 1) {
+      await new Promise((resolve) => setTimeout(resolve, 0));
+    }
+  }
+
+  /** Moves virtual time forward, firing every timer that falls due, in order. */
+  async function advance(ms: number): Promise<void> {
+    const end = clock + ms;
+    for (;;) {
+      const due = [...timers]
+        .filter(([, timer]) => timer.at <= end)
+        .sort((left, right) => left[1].at - right[1].at)[0];
+      if (!due) {
+        break;
+      }
+      timers.delete(due[0]);
+      clock = due[1].at;
+      due[1].run();
+      await settle();
+    }
+    clock = end;
+  }
+
+  function build() {
+    return createDisplayController({
+      clearTimer: (handle) => timers.delete(handle as number),
+      context: host.context,
+      decodeImage: () => Promise.resolve(),
+      media: createMemoryMediaCache(),
+      newViewerKey: () => "viewer-fixed-key-0001",
+      now: () => clock,
+      root,
+      setTimer: (run, delayMs) => {
+        const id = nextTimer;
+        nextTimer += 1;
+        timers.set(id, { at: clock + delayMs, run });
+        return id;
+      },
+      store: local,
+    });
+  }
+
+  /** One ordinary live run, so the display has a persisted snapshot to continue from, exactly as a
+   * screen that has been on the wall for a while does. */
+  async function runOnceLive(): Promise<void> {
+    const display = build();
+    await display.start();
+    await settle();
+    expect(root.dataset.offline).toBe("false");
+    await display.stop();
+    timers.clear();
+  }
+
+  it("registers once the server answers, after its first registration failed", async () => {
+    await runOnceLive();
+    server.sessionFailures = 1;
+    const sessionsBefore = server.sessionRequests;
+
+    const display = build();
+    await display.start();
+    await settle();
+    // Continuing from its own state while it cannot register: the honest banner.
+    expect(root.dataset.offline).toBe("true");
+
+    await advance(60_000);
+
+    expect(server.sessionRequests - sessionsBefore).toBeGreaterThanOrEqual(2);
+    expect(root.dataset.offline).toBe("false");
+    expect(subscriptions.length).toBeGreaterThanOrEqual(2);
+    await display.stop();
+  });
+
+  it("keeps retrying registration with backoff, and never gives up", async () => {
+    await runOnceLive();
+    server.sessionFailures = 8;
+    const sessionsBefore = server.sessionRequests;
+
+    const display = build();
+    await display.start();
+    await settle();
+    // Eight failures back off for at most 1 + 2 + 4 + 8 + 16 + 32 + 60 + 60 seconds, the last two at
+    // the one-minute ceiling. Five virtual minutes is enough for all of them and the success after.
+    await advance(5 * 60_000);
+
+    expect(server.sessionRequests - sessionsBefore).toBe(9);
+    expect(root.dataset.offline).toBe("false");
+    await display.stop();
+  });
+
+  it("retries a failed state refresh until the server answers, with no realtime nudge", async () => {
+    const display = build();
+    await display.start();
+    await settle();
+    expect(root.dataset.offline).toBe("false");
+
+    // The slot's deadline refresh fails, as it would mid-restart. Realtime says nothing.
+    server.stateFailing = true;
+    await advance(6_000);
+    expect(root.dataset.offline).toBe("true");
+    const failedRequests = server.stateRequests;
+
+    server.stateFailing = false;
+    await advance(120_000);
+
+    expect(server.stateRequests).toBeGreaterThan(failedRequests);
+    expect(root.dataset.offline).toBe("false");
+    await display.stop();
+  });
+
+  it("subscribes again when the host reports its realtime stream closed for good", async () => {
+    const display = build();
+    await display.start();
+    await settle();
+    expect(subscriptions).toHaveLength(1);
+
+    subscriptions[0]?.onStatus?.("closed");
+    await settle();
+    await advance(60_000);
+
+    expect(subscriptions.length).toBeGreaterThanOrEqual(2);
+    await display.stop();
+  });
+
+  it("schedules nothing after it is stopped", async () => {
+    await runOnceLive();
+    server.sessionFailures = 3;
+    const display = build();
+    await display.start();
+    await settle();
+    await display.stop();
+    const sessionsAtStop = server.sessionRequests;
+
+    await advance(5 * 60_000);
+
+    expect(server.sessionRequests).toBe(sessionsAtStop);
+  });
+});

tests/browser/display.test.ts

@@ -6,6 +6,7 @@ import {
   type DisplayChannelStateView,
   type DisplayController,
 } from "../../src/display/controller.js";
+import type { ViewerSessionResponse } from "../../src/shared/display-protocol.js";
 import { createFakeHost, jsonResponse } from "./support/fake-host.js";
 
 /**
@@ -114,12 +115,14 @@ function state(overrides: Partial<DisplayChannelStateView> = {}): DisplayChannel
   };
 }
 
-const SESSION = {
+/** Typed as the shared protocol message, so a stub cannot drift from what the server sends. */
+const SESSION: ViewerSessionResponse = {
   channelId: "chan-1",
   channelKey: "default",
   groupId: "group-1",
   heartbeatIntervalMs: 12_000,
   viewerId: "11111111-1111-4111-8111-111111111111",
+  viewerName: "Foyer",
 };
 
 describe("display controller", () => {

tests/browser/offline.test.ts

@@ -16,6 +16,7 @@ import {
   createDisplayController,
   type DisplayChannelStateView,
 } from "../../src/display/controller.js";
+import type { DisplaySelection, ViewerSessionResponse } from "../../src/shared/display-protocol.js";
 import { createFakeHost, jsonResponse } from "./support/fake-host.js";
 
 /**
@@ -309,14 +310,26 @@ describe("a display that loses its network", () => {
     }
   }
 
-  function nativeSelection(id: string, slotId: string): SceneSelection {
+  /** A native selection exactly as the server sends one in channel state. */
+  function nativeSelection(id: string, slotId: string): DisplaySelection {
     return {
       ...selection({ id, slotId }),
       media: {
-        display: { url: `https://cdn.invalid/${id}?sig=a` },
+        display: {
-        thumbnail: { url: `https://cdn.invalid/${id}-t?sig=a` },
+          expiresAtMs: 900_000,
+          height: 1080,
+          url: `https://cdn.invalid/${id}?sig=a`,
+          width: 1920,
         },
-    } as SceneSelection;
+        thumbnail: {
+          expiresAtMs: 900_000,
+          height: 270,
+          url: `https://cdn.invalid/${id}-t?sig=a`,
+          width: 480,
+        },
+      },
+      stale: false,
+    };
   }
 
   function state(currentId: string): DisplayChannelStateView {
@@ -373,7 +386,8 @@ describe("a display that loses its network", () => {
               groupId: "group-1",
               heartbeatIntervalMs: 12_000,
               viewerId: "11111111-1111-4111-8111-111111111111",
-            }),
+              viewerName: "Foyer",
+            } satisfies ViewerSessionResponse),
         },
         {
           method: "GET",

tests/display-protocol.test.ts

@@ -0,0 +1,125 @@
+import { describe, expect, it } from "vitest";
+import {
+  DISPLAY_ERROR_DETAIL_MAX,
+  parseDisplayHeartbeat,
+  parseViewerSessionRequest,
+  parseViewerSessionResponse,
+} from "../src/shared/display-protocol.js";
+
+/**
+ * The rules every display message follows, pinned one by one, because the production failure was
+ * exactly a disagreement about one of them.
+ */
+
+const FULL_HEARTBEAT = {
+  cachedAssetCount: 3,
+  error: null,
+  offlineReady: true,
+  reportedContributionId: null,
+  reportedPlaybackRevision: 4,
+  reportedProviderId: "signage.native-images",
+  reportedSelectionId: "sel-1",
+  reportedTimelineId: "timeline-1",
+};
+
+describe("display heartbeat", () => {
+  it("accepts null for every nullable field: null is a value, not an omission", () => {
+    const result = parseDisplayHeartbeat({
+      ...FULL_HEARTBEAT,
+      reportedContributionId: null,
+      reportedPlaybackRevision: null,
+      reportedProviderId: null,
+      reportedSelectionId: null,
+      reportedTimelineId: null,
+    });
+    expect(result).toEqual({
+      ok: true,
+      value: {
+        ...FULL_HEARTBEAT,
+        reportedContributionId: null,
+        reportedPlaybackRevision: null,
+        reportedProviderId: null,
+        reportedSelectionId: null,
+        reportedTimelineId: null,
+      },
+    });
+  });
+
+  it("treats an absent field as not reported, so the receiver keeps what it had", () => {
+    const result = parseDisplayHeartbeat({ offlineReady: false });
+    expect(result).toEqual({ ok: true, value: { offlineReady: false } });
+  });
+
+  it("ignores a field it does not know, so a newer display is never refused by an older server", () => {
+    const result = parseDisplayHeartbeat({ ...FULL_HEARTBEAT, reportedSomethingNew: { a: 1 } });
+    expect(result).toEqual({ ok: true, value: FULL_HEARTBEAT });
+  });
+
+  it.each([
+    ["cachedAssetCount", null],
+    ["cachedAssetCount", -1],
+    ["cachedAssetCount", 1.5],
+    ["offlineReady", null],
+    ["offlineReady", "yes"],
+    ["reportedSelectionId", ""],
+    ["reportedSelectionId", "x".repeat(65)],
+    ["reportedProviderId", 7],
+    ["reportedPlaybackRevision", -1],
+    ["error", { code: "" }],
+    ["error", { code: "BAD CODE" }],
+    ["error", { code: "OK", detail: "y".repeat(DISPLAY_ERROR_DETAIL_MAX + 1) }],
+    ["error", "SLIDE_UNRENDERABLE"],
+  ])("refuses %s = %j, naming that field and nothing else", (field, value) => {
+    expect(parseDisplayHeartbeat({ ...FULL_HEARTBEAT, [field]: value })).toEqual({
+      field,
+      ok: false,
+    });
+  });
+
+  it("accepts a reported error with or without detail", () => {
+    expect(parseDisplayHeartbeat({ error: { code: "SLIDE_UNRENDERABLE", detail: null } })).toEqual({
+      ok: true,
+      value: { error: { code: "SLIDE_UNRENDERABLE", detail: null } },
+    });
+    expect(parseDisplayHeartbeat({ error: { code: "SLIDE_UNRENDERABLE" } })).toEqual({
+      ok: true,
+      value: { error: { code: "SLIDE_UNRENDERABLE", detail: null } },
+    });
+  });
+
+  it("refuses something that is not an object at all", () => {
+    for (const body of [null, [], "heartbeat", 3]) {
+      expect(parseDisplayHeartbeat(body)).toEqual({ field: "body", ok: false });
+    }
+  });
+});
+
+describe("viewer session", () => {
+  it("requires a well-formed viewer key", () => {
+    expect(parseViewerSessionRequest({ viewerKey: "kiosk-lobby-0001" })).toEqual({
+      ok: true,
+      value: { viewerKey: "kiosk-lobby-0001" },
+    });
+    for (const viewerKey of [undefined, null, "short", "has spaces in it", "x".repeat(129)]) {
+      expect(parseViewerSessionRequest({ viewerKey })).toEqual({ field: "viewerKey", ok: false });
+    }
+  });
+
+  it("a display accepts only a complete registration", () => {
+    const complete = {
+      channelId: "chan-1",
+      channelKey: "default",
+      groupId: "group-1",
+      heartbeatIntervalMs: 12_000,
+      viewerId: "11111111-1111-4111-8111-111111111111",
+      viewerName: "New display",
+    };
+    expect(parseViewerSessionResponse(complete)).toEqual({ ok: true, value: complete });
+    const { viewerId: _omitted, ...missing } = complete;
+    expect(parseViewerSessionResponse(missing)).toEqual({ field: "viewerId", ok: false });
+    expect(parseViewerSessionResponse({ ...complete, heartbeatIntervalMs: 0 })).toEqual({
+      field: "heartbeatIntervalMs",
+      ok: false,
+    });
+  });
+});