add test, update docs

packages/playwright/src/util.ts

@@ -319,8 +319,16 @@ export function resolveImportSpecifierExtension(resolved: string, isPathMapping:
     break;  // Do not try '' when a more specific extension like '.jsx' matched.
   }
 
-  // Following TypeScript's path mapping logic, index files and package.json are not resolved in ESM.
-  // TypeScript does not interpret package.json for path mappings: https://www.typescriptlang.org/docs/handbook/modules/reference.html#paths-should-not-point-to-monorepo-packages-or-node_modules-packages
+  // After TypeScript path mapping, here's how directories with a `package.json` are resolved:
+  //  - `package.json#exports` is not respected
+  //  - `package.json#main` is respected only in CJS mode
+  //  - `index.js` default is respected only in CJS mode
+  //
+  // More info:
+  //  - https://www.typescriptlang.org/docs/handbook/modules/reference.html#paths-should-not-point-to-monorepo-packages-or-node_modules-packages
+  //  - https://www.typescriptlang.org/docs/handbook/modules/reference.html#directory-modules-index-file-resolution
+  //  - https://nodejs.org/dist/latest-v20.x/docs/api/modules.html#folders-as-modules
+
   const shouldNotResolveDirectory = isPathMapping && isESM;
 
   if (!shouldNotResolveDirectory && dirExists(resolved)) {

tests/playwright-test/resolver.spec.ts

@@ -606,6 +606,42 @@ test('should import packages with non-index main script through path resolver',
   expect(result.output).toContain(`foo=42`);
 });
 
+test('should not honor `package.json#main` field in ESM mode', async ({ runInlineTest }) => {
+  const result = await runInlineTest({
+    'app/pkg/main.ts': `
+      export const foo = 42;
+    `,
+    'app/pkg/package.json': `
+      { "main": "main.ts" }
+    `,
+    'package.json': `
+      { "name": "example-project", "type": "module" }
+    `,
+    'playwright.config.ts': `
+      export default {};
+    `,
+    'tsconfig.json': `{
+      "compilerOptions": {
+        "baseUrl": ".",
+        "paths": {
+          "app/*": ["app/*"],
+        },
+      },
+    }`,
+    'example.spec.ts': `
+      import { foo } from 'app/pkg';
+      import { test, expect } from '@playwright/test';
+      test('test', ({}) => {
+        console.log('foo=' + foo);
+      });
+    `,
+  });
+
+  expect(result.exitCode).toBe(1);
+  expect(result.output).toContain(`Cannot find package 'app'`);
+});
+
+
 test('does not honor `exports` field after type mapping', async ({ runInlineTest }) => {
   const result = await runInlineTest({
     'app/pkg/main.ts': `