diff --git a/test/platform-detection.test.ts b/test/platform-detection.test.ts
new file mode 100644
index 00000000000..68ec4a7235b
--- /dev/null
+++ b/test/platform-detection.test.ts
@@ -0,0 +1,44 @@
+import { isMacOS, isFirefox, isIOS } from "../app/utils";
+
+// These helpers read navigator.userAgent, so we temporarily override it and
+// restore the original descriptor afterwards to keep the test side-effect free.
+const originalUA = Object.getOwnPropertyDescriptor(
+  window.navigator,
+  "userAgent",
+);
+
+function setUserAgent(ua: string) {
+  Object.defineProperty(window.navigator, "userAgent", {
+    value: ua,
+    configurable: true,
+  });
+}
+
+afterAll(() => {
+  if (originalUA) {
+    Object.defineProperty(window.navigator, "userAgent", originalUA);
+  }
+});
+
+describe("platform detection", () => {
+  test("isMacOS is true for a Macintosh UA and false otherwise", () => {
+    setUserAgent("Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7)");
+    expect(isMacOS()).toBe(true);
+    setUserAgent("Mozilla/5.0 (Windows NT 10.0; Win64; x64)");
+    expect(isMacOS()).toBe(false);
+  });
+
+  test("isIOS is true for iPhone/iPad UAs and false otherwise", () => {
+    setUserAgent("Mozilla/5.0 (iPhone; CPU iPhone OS 16_0 like Mac OS X)");
+    expect(isIOS()).toBe(true);
+    setUserAgent("Mozilla/5.0 (Windows NT 10.0; Win64; x64)");
+    expect(isIOS()).toBe(false);
+  });
+
+  test("isFirefox is true only for a Firefox UA", () => {
+    setUserAgent("Mozilla/5.0 (X11; Linux) Gecko/20100101 Firefox/120.0");
+    expect(isFirefox()).toBe(true);
+    setUserAgent("Mozilla/5.0 (Chrome/120) AppleWebKit/537.36");
+    expect(isFirefox()).toBe(false);
+  });
+});