It works-ish

vitest-setup-client.ts

@@ -0,0 +1,18 @@
+import "@testing-library/jest-dom/vitest";
+import { vi } from "vitest";
+
+// required for svelte5 + jsdom as jsdom does not support matchMedia
+Object.defineProperty(window, "matchMedia", {
+  writable: true,
+  enumerable: true,
+  value: vi.fn().mockImplementation((query) => ({
+    matches: false,
+    media: query,
+    onchange: null,
+    addEventListener: vi.fn(),
+    removeEventListener: vi.fn(),
+    dispatchEvent: vi.fn(),
+  })),
+});
+
+// add more mocks here if you need them