fix(clock): mock time in Event.prototype.timeStamp (microsoft#31986)

Ideally we generate the timestamp when the Event gets created. This
patch adds a best-effort logic, since we can't override the constructor
of natively created events, e.g. `MouseEvent`.

Fixes microsoft#31924

docs/src/clock.md

@@ -31,6 +31,7 @@ The recommended approach is to use `setFixedTime` to set the time to a specific
   - `requestIdleCallback`
   - `cancelIdleCallback`
   - `performance`
+  - `Event.timeStamp`
 :::
 
 ## Test with predefined time

packages/playwright-core/src/server/injected/clock.ts

@@ -697,6 +697,14 @@ export function install(globalObject: WindowOrWorkerGlobalScope, config: Install
       (globalObject as any).Intl = api[method]!;
     } else if (method === 'performance') {
       (globalObject as any).performance = api[method]!;
+      const kEventTimeStamp = Symbol('playwrightEventTimeStamp');
+      Object.defineProperty(Event.prototype, 'timeStamp', {
+        get() {
+          if (!this[kEventTimeStamp])
+            this[kEventTimeStamp] = api.performance?.now();
+          return this[kEventTimeStamp];
+        }
+      });
     } else {
       (globalObject as any)[method] = (...args: any[]) => {
         return (api[method] as any).apply(api, args);

tests/library/clock.spec.ts

@@ -1060,6 +1060,18 @@ it.describe('stubTimers', () => {
     expect(prev).toBe(0);
   });
 
+  it('replace Event.prototype.timeStamp', async ({ install }) => {
+    it.info().annotations.push({ type: 'issue', description: 'https://github.com/microsoft/playwright/issues/31924' });
+    const clock = install();
+    await clock.runFor(1000);
+    const event1 = new Event('foo');
+    expect(event1.timeStamp).toBe(1000);
+    await clock.runFor(1000);
+    const event2 = new Event('foo');
+    expect(event2.timeStamp).toBe(2000);
+    expect(event1.timeStamp).toBe(1000);
+  });
+
   it('uninstalls global performance.now', async ({ install }) => {
     const oldNow = performance.now;
     const clock = install();