fix(display): keep blackout across reloads and follow the display's current assignment

A kiosk that reloaded during a blackout restored its last frame from local
state without the blackout, so the hidden slide flashed on the wall until the
server answered.

A display resolved its channel once, at registration. Moving it to another
group or audience, or deleting the channel it was on, never reached a running
screen: it kept showing the old channel, or fell into offline continuation
against a 404 forever. The display now fetches its state by viewer, so the
server resolves the current assignment on every fetch, and the heartbeat reply
names the assigned channel so a reassignment lands within one heartbeat. A
display removed from the roster registers again once, on backoff; only
Signage's own not-found means removal, so a foreign 404 can never cause a
registration loop. The channel route stays for the preview and for display
code a kiosk cached earlier. A replaced realtime subscription can no longer
schedule a stray resubscription when it reports closing.

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

tests/browser/display-contract.test.ts

@@ -200,7 +200,8 @@ describe("display ↔ server contract", () => {
 
     const sent = calls.map((call) => `${call.method} ${call.path}`);
     expect(sent).toContain("POST /viewers/session");
-    expect(sent).toContain("GET /channels/:channelId/display-state");
+    // By viewer, so the server — not the display's memory of its registration — decides the channel.
+    expect(sent).toContain("GET /viewers/:viewerId/display-state");
     expect(sent).toContain("POST /viewers/:viewerId/heartbeat");
     // Not one refusal, of any kind, for anything the display built itself.
     expect(calls.filter((call) => call.status >= 400)).toEqual([]);
@@ -220,4 +221,48 @@ describe("display ↔ server contract", () => {
     expect(viewer?.reportedProviderId).toBe(SIGNAGE_NATIVE_PROVIDER_ID);
     expect(viewer?.reportedContributionId).toBeNull();
   });
+
+  it("a display moved to another channel group is served that group's state on its next fetch", async () => {
+    const display = controller();
+    await display.start();
+    await settle();
+    const viewerId = display.diagnostics().viewerId;
+    const firstChannel = display.diagnostics().channelId;
+    expect(viewerId).toBeDefined();
+
+    // An administrator creates a second group and moves this screen to it while it is running.
+    const groupId = newId();
+    const channelId = newId();
+    await store.createChannelGroup({ channelId, channelName: "Default", groupId, name: "Nursery" });
+    await store.updateViewer(viewerId as string, { channelId });
+
+    const response = await host.context.api.request(`viewers/${viewerId}/display-state`);
+    const state = (await response.json()) as { channel: { id: string; groupId: string } };
+    expect(state.channel).toMatchObject({ groupId, id: channelId });
+    expect(channelId).not.toBe(firstChannel);
+
+    // And the heartbeat names the new assignment, which is how a display with a long slot hears.
+    const beat = await host.context.api.request(`viewers/${viewerId}/heartbeat`, {
+      body: JSON.stringify({}),
+      headers: { "content-type": "application/json" },
+      method: "POST",
+    });
+    expect(((await beat.json()) as { channelId?: string }).channelId).toBe(channelId);
+    await display.stop();
+  });
+
+  it("a display whose channel was deleted falls back to the default channel, not to nothing", async () => {
+    const display = controller();
+    await display.start();
+    await settle();
+    const viewerId = display.diagnostics().viewerId as string;
+    await store.updateViewer(viewerId, { channelId: null });
+
+    const response = await host.context.api.request(`viewers/${viewerId}/display-state`);
+    expect(response.status).toBe(200);
+    const [group] = await store.listChannelGroups();
+    const fallback = group?.channels.find((channel) => channel.isDefault);
+    expect(((await response.json()) as { channel: { id: string } }).channel.id).toBe(fallback?.id);
+    await display.stop();
+  });
 });