fix(test-runner-core): await browser cleanup in watch mode

.changeset/quiet-browsers-wait.md

@@ -0,0 +1,5 @@
+---
+'@web/test-runner-core': patch
+---
+
+Wait for browsers to finish closing before completing test runner shutdown in watch mode.

packages/test-runner-core/src/runner/TestRunner.ts

@@ -166,7 +166,6 @@ export class TestRunner extends EventEmitter<EventMap> {
     if (this.config.watch) {
       // we only need to stop the browsers in watch mode, in non-watch
       // mode the scheduler has already stopped them
-      const stopActions = [];
       for (const browser of this.browsers) {
         if (browser.stop) {
           stopActions.push(

packages/test-runner-core/test/src/runner/TestRunner.test.ts

@@ -1,6 +1,7 @@
 import assert from 'node:assert/strict';
 import path from 'node:path';
 import { describe, it, mock } from 'node:test';
+import { setImmediate } from 'node:timers/promises';
 
 import portfinder from 'portfinder';
 
@@ -9,6 +10,7 @@ import type { TestRunnerCoreConfig } from '../../../dist/config/TestRunnerCoreCo
 import type { TestRunnerGroupConfig } from '../../../dist/index.js';
 import type { Logger } from '../../../dist/logger/Logger.js';
 import { TestRunner } from '../../../dist/runner/TestRunner.js';
+import { TestRunnerServer } from '../../../dist/server/TestRunnerServer.js';
 import { SESSION_STATUS } from '../../../dist/test-session/TestSessionStatus.js';
 
 function createBrowserStub(): BrowserLauncher {
@@ -134,6 +136,39 @@ describe('TestRunner', function () {
     assert.equal(passed, false, 'test runner quits with false');
   });
 
+  it('waits for browser cleanup in watch mode', async t => {
+    const { runner, browser } = await createTestRunner({ watch: true });
+    const stopServer = t.mock.method(TestRunnerServer.prototype, 'stop');
+    const browserStopped = Promise.withResolvers<void>();
+    const stopped = mock.fn();
+    runner.on('stopped', stopped);
+    browser.stop.mock.mockImplementation(() => browserStopped.promise);
+
+    await runner.start();
+
+    let stopResolved = false;
+    const stopping = runner.stop().then(() => {
+      stopResolved = true;
+    });
+
+    try {
+      await setImmediate();
+      assert.equal(stopServer.mock.callCount(), 1);
+      await stopServer.mock.calls[0].result;
+      await setImmediate();
+
+      assert.equal(browser.stop.mock.callCount(), 1, 'browser cleanup has started');
+      assert.equal(stopped.mock.callCount(), 0, 'stopped event waits for browser cleanup');
+      assert.equal(stopResolved, false, 'stop waits for browser cleanup');
+    } finally {
+      browserStopped.resolve();
+      await stopping;
+    }
+
+    assert.equal(stopped.mock.callCount(), 1, 'stopped event is emitted after browser cleanup');
+    assert.equal(stopResolved, true);
+  });
+
   describe('groups', () => {
     it('can create a group in addition to the default group', async () => {
       const { runner } = await createTestRunner(undefined, [