docs(chrome-extensions-js-python.md) add headless testing docs (#19812)

See https://github.com/microsoft/playwright/issues/19233

docs/src/chrome-extensions-js-python.md

@@ -4,7 +4,7 @@ title: "Chrome Extensions"
 ---
 
 :::note
-Extensions only work in Chrome / Chromium in non-headless mode, launched with a persistent context.
+Extensions only work in Chrome / Chromium launched with a persistent context.
 :::
 
 The following is code for getting a handle to the [background page](https://developer.chrome.com/extensions/background_pages) of a [Manifest v2](https://developer.chrome.com/docs/extensions/mv2/) extension whose source is located in `./my-extension`:
@@ -212,3 +212,36 @@ def test_popup_page(page: Page, extension_id: str) -> None:
     page.goto(f"chrome-extension://{extension_id}/popup.html")
     expect(page.locator("body")).to_have_text("my-extension popup")
 ```
+
+## Headless mode
+
+By default, Chrome's headless mode in Playwright does not support Chrome extensions. To overcome this limitation, you can run Chrome's persistent context with a new headless mode by using the following code:
+```ts
+// fixtures.ts
+// ...
+
+const pathToExtension = path.join(__dirname, 'my-extension');
+const context = await chromium.launchPersistentContext('', {
+  headless: false,
+  args: [
+    `--headless=chrome`, // the new headless arg
+    `--disable-extensions-except=${pathToExtension}`,
+    `--load-extension=${pathToExtension}`,
+  ],
+});
+// ...
+```
+
+```python
+# conftest.py
+path_to_extension = Path(__file__).parent.joinpath("my-extension")
+context = playwright.chromium.launch_persistent_context(
+    "",
+    headless=False, 
+    args=[
+        "--headless=chrome", # the new headless arg
+        f"--disable-extensions-except={path_to_extension}",
+        f"--load-extension={path_to_extension}",
+    ],
+)
+```