Extract the spotlight speaker choice into a pure function

src/state/layoutMedia.test.ts

@@ -0,0 +1,62 @@
+/*
+Copyright 2026 Element Creations Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { describe, expect, it } from "vitest";
+
+import { chooseSpotlightSpeaker } from "./layoutMedia";
+import { type LocalUserMediaViewModel } from "./media/LocalUserMediaViewModel";
+import { type RemoteUserMediaViewModel } from "./media/RemoteUserMediaViewModel";
+
+// The layout functions only look at `type` and `local`, so stubs suffice
+const local = { type: "user", local: true } as LocalUserMediaViewModel;
+const alice = { type: "user", local: false } as RemoteUserMediaViewModel;
+const bob = { type: "user", local: false } as RemoteUserMediaViewModel;
+const carol = { type: "user", local: false } as RemoteUserMediaViewModel;
+
+describe("chooseSpotlightSpeaker", () => {
+  const items = (
+    ...speaking: boolean[]
+  ): {
+    media: RemoteUserMediaViewModel | LocalUserMediaViewModel;
+    speaking: boolean;
+  }[] =>
+    [local, alice, bob].map((media, i) => ({
+      media,
+      speaking: speaking[i] ?? false,
+    }));
+
+  it("prefers a remote speaker over the local user", () => {
+    expect(chooseSpotlightSpeaker(undefined, items(true, false, true))).toBe(
+      bob,
+    );
+  });
+
+  it("falls back to any remote user, then the local user", () => {
+    expect(chooseSpotlightSpeaker(undefined, items())).toBe(alice);
+    expect(
+      chooseSpotlightSpeaker(undefined, [{ media: local, speaking: false }]),
+    ).toBe(local);
+    expect(chooseSpotlightSpeaker(undefined, [])).toBeUndefined();
+  });
+
+  it("sticks with the previous speaker while they keep speaking", () => {
+    expect(chooseSpotlightSpeaker(bob, items(false, true, true))).toBe(bob);
+  });
+
+  it("sticks with the previous speaker when nobody is speaking", () => {
+    expect(chooseSpotlightSpeaker(bob, items())).toBe(bob);
+  });
+
+  it("switches when someone else speaks and the previous speaker is silent", () => {
+    expect(chooseSpotlightSpeaker(bob, items(false, true, false))).toBe(alice);
+  });
+
+  it("forgets a previous speaker who left, and never sticks to the local user", () => {
+    expect(chooseSpotlightSpeaker(carol, items())).toBe(alice);
+    expect(chooseSpotlightSpeaker(local, items())).toBe(alice);
+  });
+});