[BUG] Javascript __doPostBack missing in browser opened by playwright.

[quldude]

System info

• Playwright Version: v1.31.2
• Operating System: All
• Browser: All

Source code

• I provided exact source code that allows reproducing the issue locally.

Config file

// playwright.config.ts
const config: PlaywrightTestConfig = {
  testDir: '.',
  /* Maximum time one test can run for. */
  timeout: 300 * 1000,
  expect: {
    /**
     * Maximum time expect() should wait for the condition to be met.
     * For example in `await expect(locator).toHaveText();`
     */
    timeout: 5 * 1000,
  },
  /* Run tests in files in parallel */
  fullyParallel: true,
  /* Fail the build on CI if you accidentally left test.only in the source code. */
  forbidOnly: !!process.env.CI,
  retries: 0,
  /* Opt out of parallel tests on CI. */
  workers: process.env.CI ? 1 : undefined,
  /* Reporter to use. See https://playwright.dev/docs/test-reporters */
  reporter: 'html',
  globalSetup: require.resolve('./lib/global-setup'),
  /* Shared settings for all the projects below. See https://playwright.dev/docs/api/class-testoptions. */
  use: {
    headless: false,
    // channel: 'msedge',

    /* Maximum time each action such as `click()` can take. Defaults to 0 (no limit). */
    actionTimeout: 0,
    /* Base URL to use in actions like `await page.goto('/')`. */
    baseURL: process.env.CI ? process.env.TEST_SERVER_URL : 'http://localhost:3000',

    /* Use data-qa as the default test-id attribute */
    testIdAttribute: 'data-qa',

    /* Collect trace when retrying the failed test. See https://playwright.dev/docs/trace-viewer */
    trace: 'on',

    // This is not read when projects section is defined. Ref: https://github.com/microsoft/playwright/issues/16077
    userAgent: customUserAgent,
  },

  /* Folder for test artifacts such as screenshots, videos, traces, etc. */
  outputDir: 'test-results/',
};

Test file (self-contained)

await test.step('Navigate to url.', async () => {
    await page.goto(
      'https://some.url',
    );
  });

Steps

• Run playwright test that opens browser and navigates to a specific page.

Expected

Page html should have following script:

<script type="text/javascript">
//<![CDATA[
var theForm = document.forms['aspnetForm'];
if (!theForm) {
    theForm = document.aspnetForm;
}
function __doPostBack(eventTarget, eventArgument) {
    if (!theForm.onsubmit || (theForm.onsubmit() != false)) {
        theForm.__EVENTTARGET.value = eventTarget;
        theForm.__EVENTARGUMENT.value = eventArgument;
        theForm.submit();
    }
}
//]]>
</script>

Actual

Page is missing the expected javascript. So we get error:

Uncaught ReferenceError: __doPostBack is not defined
    at HTMLButtonElement.onclick

This only happens in the browser that is opened by playwright. Wanted to see if there is some setting I'm missing.

[quldude]

Update-1: the issue is happening because of userAgent defined in config. If I remove it, then it works.

[quldude]

Update-2: I was able to resolve this by appending the custom user agent based on convo in here.