Version

1.59.1

Steps to reproduce

1. Clone the repo: https://github.com/ritz078/pw-webkit-formdata-repro
2. npm install
3. npx playwright install
4. npx playwright test

The test constructs a trivial multipart/form-data Response and calls .formData() on it:

const boundary = '----WebKitFormBoundary1234';
const body = [
  `--${boundary}`,
  'Content-Disposition: form-data; name="field1"',
  '',
  'value1',
  `--${boundary}`,
  'Content-Disposition: form-data; name="file1"; filename="test.png"',
  'Content-Type: image/png',
  '',
  'PNG_BINARY_DATA',
  `--${boundary}--`,
].join('\r\n');

const response = new Response(body, {
  headers: { 'Content-Type': `multipart/form-data; boundary=${boundary}` },
});

const fd = await response.formData(); // TypeError: Type error (WebKit only)

Expected behavior

Response.formData() should parse the multipart body and return a FormData object with the expected keys, as it does on Chromium and Firefox.

Actual behavior

On WebKit, Response.formData() throws TypeError: Type error for any multipart/form-data response — even trivially simple ones.

  ✓ [chromium] › Response.formData() works with multipart/form-data
  ✘ [webkit]  › Response.formData() works with multipart/form-data
  ✓ [firefox] › Response.formData() works with multipart/form-data

  Error: TypeError: Type error

Additional context

• Regression: This works in Playwright 1.58.0 (WebKit 26.0) but fails in 1.59.0 (WebKit 26.4).
• The native FormData constructor and new Request(...).formData() are unaffected — only Response.formData() is broken.
• This affects any application that uses fetch() and then calls .formData() on the response to parse multipart data from a server.