fix(player): a fullscreen button the browser cannot honour is not shown

Brave on iPadOS switches element fullscreen off, patches requestFullscreen with
a video-only stand-in and fakes fullscreenEnabled, so the Plex fullscreen button
and the YouTube docked bar's did nothing. Support is now read off
Element.prototype and the buttons are left out without it (owner). A stand-in
that returns a boolean no longer throws. A test's bare matchMedia stub no
longer outlives its describe.

frontend/src/components/PlayerModal.dom.test.tsx

@@ -551,8 +551,19 @@ describe("PlayerModal — the side panel on a wide window", () => {
           removeEventListener: () => {},
         }) as unknown as MediaQueryList,
     );
+  // jsdom has no matchMedia to spy on: a bare one is put in for these tests and taken out after, or
+  // every later test's media query would read that bare stub.
+  let added = false;
   beforeEach(() => {
-    window.matchMedia ??= (() => ({})) as unknown as typeof window.matchMedia;
+    if (typeof window.matchMedia === "function") return;
+    window.matchMedia = (() => ({})) as unknown as typeof window.matchMedia;
+    added = true;
+  });
+  afterEach(() => {
+    // The spy goes first: restoring it later would put the bare stub back.
+    vi.restoreAllMocks();
+    if (added) delete (window as { matchMedia?: unknown }).matchMedia;
+    added = false;
   });
 
   it("puts the details beside the video, the description open, and sizes the card for both", async () => {
@@ -657,3 +668,33 @@ describe("PlayerModal — the side panel on a wide window", () => {
     expect(screen.queryByTestId("player-description")).toBeNull();
   });
 });
+
+// Brave on an iPad cannot fullscreen an element (it fakes `fullscreenEnabled`): the docked bar's
+// true-fullscreen button did nothing there. The window-filling view is then the biggest one.
+describe("PlayerModal — the docked bar's fullscreen", () => {
+  const maximised = (api_: boolean) => {
+    if (api_)
+      Object.defineProperty(Element.prototype, "requestFullscreen", {
+        configurable: true,
+        value: () => Promise.resolve(),
+      });
+    vi.spyOn(api, "videoDetail").mockReturnValue(new Promise(() => {}));
+    const A = vid("A", 0);
+    renderModal(
+      <PlayerModal video={A} queue={[A]} startIndex={0} onClose={() => {}} onState={() => {}} />,
+    );
+    fireEvent.click(screen.getByLabelText("player.fullscreen"));
+    return screen.getAllByLabelText("player.fullscreen").length;
+  };
+  afterEach(() => {
+    delete (Element.prototype as { requestFullscreen?: unknown }).requestFullscreen;
+  });
+
+  it("is offered where the browser can fullscreen an element", () => {
+    expect(maximised(true)).toBe(2);
+  });
+
+  it("is left out where it cannot", () => {
+    expect(maximised(false)).toBe(1);
+  });
+});