test: mock resize observer and element.animate

vitest.setup.ts

@@ -11,6 +11,24 @@ global.TextEncoder = TextEncoder;
   global as { IntersectionObserver: typeof IntersectionObserver }
 ).IntersectionObserver = IntersectionObserverPassive;
 
+// We mock ResizeObserver and element.animate because neither JSDOM nor Happy DOM supports them, while Svelte v5 requires them.
+// Interesting related thread: https://github.com/testing-library/svelte-testing-library/issues/284
+global.ResizeObserver = class ResizeObserver {
+  observe() {
+    // do nothing
+  }
+  unobserve() {
+    // do nothing
+  }
+  disconnect() {
+    // do nothing
+  }
+};
+
+Element.prototype.animate = vi
+  .fn()
+  .mockImplementation(() => ({ cancel: vi.fn(), finished: Promise.resolve() }));
+
 // testing-library setup
 configure({
   testIdAttribute: "data-tid",