chore: Upgrade to Vitest 4 and regroup tests for performance (#2618)

tests/utils/basic.test.ts

@@ -32,7 +32,7 @@ import {
   setOrRemoveAttribute,
   setOrRemoveStyleProperty,
 } from '../../src/utils/basic.js';
-import { createSlot, createSlotHost } from '../test-utils.js';
+import { createSlot, createSlotHost, stubMatchMedia } from '../test-utils.js';
 
 describe('prettifyTitle', () => {
   it('should return undefined when passed undefined', () => {
@@ -146,11 +146,11 @@ describe('isHoverableDevice', () => {
   });
 
   it('should return hoverable', () => {
-    vi.spyOn(window, 'matchMedia').mockReturnValue(<MediaQueryList>{ matches: true });
+    stubMatchMedia().mockReturnValue(<MediaQueryList>{ matches: true });
     expect(isHoverableDevice()).toBeTruthy();
   });
   it('should return not hoverable', () => {
-    vi.spyOn(window, 'matchMedia').mockReturnValue(<MediaQueryList>{ matches: false });
+    stubMatchMedia().mockReturnValue(<MediaQueryList>{ matches: false });
     expect(isHoverableDevice()).toBeFalsy();
   });
 });

tests/utils/thumbnail.test.ts

@@ -71,7 +71,11 @@ describe('thumbnail utilities', () => {
     });
     vi.stubGlobal(
       'FileReader',
-      vi.fn(() => mockFileReader),
+      // The source calls `new FileReader()`, and a mock implementation must be
+      // callable with `new`, so it cannot be an arrow function.
+      vi.fn(function () {
+        return mockFileReader;
+      }),
     );
 
     createFetchThumbnailTask(
@@ -129,7 +133,9 @@ describe('thumbnail utilities', () => {
     const mockFileReader = mock<FileReader>();
     vi.stubGlobal(
       'FileReader',
-      vi.fn(() => mockFileReader),
+      vi.fn(function () {
+        return mockFileReader;
+      }),
     );
 
     createFetchThumbnailTask(
@@ -166,7 +172,9 @@ describe('thumbnail utilities', () => {
     });
     vi.stubGlobal(
       'FileReader',
-      vi.fn(() => mockFileReader),
+      vi.fn(function () {
+        return mockFileReader;
+      }),
     );
 
     createFetchThumbnailTask(