fix: properly merge projects in defineConfig (microsoft#36898)

Author: yury-s

packages/playwright/src/common/configLoader.ts

@@ -34,6 +34,7 @@ export const defineConfig = (...configs: any[]) => {
   let result = configs[0];
   for (let i = 1; i < configs.length; ++i) {
     const config = configs[i];
+    const prevProjects = result.projects;
     result = {
       ...result,
       ...config,
@@ -63,7 +64,7 @@ export const defineConfig = (...configs: any[]) => {
       projectOverrides.set(project.name, project);
 
     const projects = [];
-    for (const project of result.projects || []) {
+    for (const project of prevProjects || []) {
       const projectOverride = projectOverrides.get(project.name);
       if (projectOverride) {
         projects.push({

tests/playwright-test/config.spec.ts

@@ -661,8 +661,8 @@ test('should merge configs', async ({ runInlineTest }) => {
         use: { foo: 1, bar: 2 },
         expect: { timeout: 12 },
         projects: [
-          { name: 'B', timeout: 40, use: {} },
-          { name: 'A', timeout: 50, use: {} }
+          { name: 'A', timeout: 50, use: {} },
+          { name: 'B', timeout: 40 },
         ],
         webServer: [{
           command: 'echo 123',
@@ -680,6 +680,33 @@ test('should merge configs', async ({ runInlineTest }) => {
   expect(result.exitCode).toBe(0);
 });
 
+test('should merge projects in the config', async ({ runInlineTest }) => {
+  const result = await runInlineTest({
+    'playwright.config.ts': `
+      import { defineConfig, expect } from '@playwright/test';
+      const baseConfig = defineConfig({
+        projects: [{ name: 'A', timeout: 5_000 }, { name: 'B', timeout: 6_000 }],
+      });
+      const derivedConfig = defineConfig(baseConfig, {
+        projects: [{ name: 'A', timeout: 7_000 }, { name: 'C', timeout: 8_000 }],
+      });
+
+      expect(derivedConfig).toEqual(expect.objectContaining({
+        projects: [
+          { name: 'A', timeout: 7_000, use: {} },
+          { name: 'B', timeout: 6_000 },
+          { name: 'C', timeout: 8_000 },
+        ],
+      }));
+    `,
+    'a.test.ts': `
+      import { test } from '@playwright/test';
+      test('pass', async ({}) => {});
+    `
+  });
+  expect(result.exitCode).toBe(0);
+});
+
 test('should merge ct configs', async ({ runInlineTest }) => {
   const result = await runInlineTest({
     'playwright.config.ts': `