diff --git a/js/timer/Timer.test.ts b/js/timer/Timer.test.ts
new file mode 100644
index 0000000000..78f96d39f1
--- /dev/null
+++ b/js/timer/Timer.test.ts
@@ -0,0 +1,73 @@
+import { afterEach, describe, expect, test, vi } from "vitest";
+import { cleanup, render } from "@self/tootils/render";
+
+import Timer from "./Index.svelte";
+
+describe("Timer", () => {
+	afterEach(() => {
+		cleanup();
+		vi.restoreAllMocks();
+	});
+
+	test("registers an interval and dispatches tick when active and visible", async () => {
+		Object.defineProperty(document, "visibilityState", {
+			configurable: true,
+			value: "visible",
+		});
+
+		let intervalCallback: (() => void) | undefined;
+		const setIntervalSpy = vi
+			.spyOn(globalThis, "setInterval")
+			.mockImplementation(((fn: TimerHandler) => {
+				intervalCallback = fn as () => void;
+				return 1 as unknown as NodeJS.Timeout;
+			}) as typeof setInterval);
+
+		const { listen } = await render(Timer, {
+			value: 0.25,
+			active: true,
+		});
+		const tick = listen("tick");
+		intervalCallback?.();
+
+		expect(setIntervalSpy).toHaveBeenCalledTimes(1);
+		expect(setIntervalSpy.mock.calls[0]?.[1]).toBe(250);
+		expect(tick).toHaveBeenCalledTimes(1);
+	});
+
+	test("does not register an interval when inactive", async () => {
+		const setIntervalSpy = vi.spyOn(globalThis, "setInterval");
+
+		await render(Timer, {
+			value: 0.25,
+			active: false,
+		});
+
+		expect(setIntervalSpy).not.toHaveBeenCalled();
+	});
+
+	test("suppresses tick dispatches while the document is hidden", async () => {
+		Object.defineProperty(document, "visibilityState", {
+			configurable: true,
+			value: "hidden",
+		});
+
+		let intervalCallback: (() => void) | undefined;
+		const setIntervalSpy = vi
+			.spyOn(globalThis, "setInterval")
+			.mockImplementation(((fn: TimerHandler) => {
+				intervalCallback = fn as () => void;
+				return 1 as unknown as NodeJS.Timeout;
+			}) as typeof setInterval);
+
+		const { listen } = await render(Timer, {
+			value: 0.25,
+			active: true,
+		});
+		const tick = listen("tick");
+		intervalCallback?.();
+
+		expect(setIntervalSpy).toHaveBeenCalledTimes(1);
+		expect(tick).not.toHaveBeenCalled();
+	});
+});