fix(game-frontend): coalesce realtime work across tabs

app/game-frontend/test/broadcastTabCoordinator.test.ts

@@ -0,0 +1,84 @@
+import assert from 'node:assert/strict';
+import { test } from 'node:test';
+import { createBroadcastTabCoordinator } from '../src/utils/broadcastTabCoordinator.ts';
+
+type FakeMessage = { kind: string; tabId: string; sentAt: number; payload?: string };
+
+class FakeBroadcastBus {
+    readonly channels = new Set<FakeBroadcastChannel>();
+
+    open(): FakeBroadcastChannel {
+        const channel = new FakeBroadcastChannel(this);
+        this.channels.add(channel);
+        return channel;
+    }
+}
+
+class FakeBroadcastChannel {
+    onmessage: ((event: MessageEvent<FakeMessage>) => void) | null = null;
+    private readonly bus: FakeBroadcastBus;
+
+    constructor(bus: FakeBroadcastBus) {
+        this.bus = bus;
+    }
+
+    postMessage(message: FakeMessage): void {
+        for (const channel of this.bus.channels) {
+            if (channel === this) continue;
+            queueMicrotask(() => channel.onmessage?.({ data: message } as MessageEvent<FakeMessage>));
+        }
+    }
+
+    close(): void {
+        this.bus.channels.delete(this);
+        this.onmessage = null;
+    }
+}
+
+const wait = (delayMs: number) => new Promise((resolve) => setTimeout(resolve, delayMs));
+
+void test('one active tab owns broadcasts, followers consume its payload, and leave hands leadership over', async () => {
+    const bus = new FakeBroadcastBus();
+    const firstLeadership: boolean[] = [];
+    const secondLeadership: boolean[] = [];
+    const firstPayloads: string[] = [];
+    const secondPayloads: string[] = [];
+    const common = {
+        createChannel: () => bus.open() as never,
+        settleMs: 5,
+        heartbeatMs: 100,
+        peerExpiryMs: 500,
+    };
+    const first = createBroadcastTabCoordinator<string>('same-account', {
+        ...common,
+        createTabId: () => 'tab-b',
+        onLeadershipChange: (leader) => firstLeadership.push(leader),
+        onPayload: (payload) => firstPayloads.push(payload),
+    });
+    const second = createBroadcastTabCoordinator<string>('same-account', {
+        ...common,
+        createTabId: () => 'tab-a',
+        onLeadershipChange: (leader) => secondLeadership.push(leader),
+        onPayload: (payload) => secondPayloads.push(payload),
+    });
+
+    first.start();
+    second.start();
+    await wait(20);
+
+    assert.equal(first.isLeader(), false);
+    assert.equal(second.isLeader(), true);
+    assert.equal(first.postFromLeader('ignored'), false);
+    assert.equal(second.postFromLeader('shared-patch'), true);
+    await wait(0);
+    assert.deepEqual(firstPayloads, ['shared-patch']);
+    assert.deepEqual(secondPayloads, []);
+
+    second.stop();
+    await wait(0);
+    assert.equal(first.isLeader(), true);
+    assert.equal(firstLeadership.at(-1), true);
+    assert.deepEqual(secondLeadership, [true, false]);
+
+    first.stop();
+});