[Bug]: Firefox not handling `keepalive: true` fetch requests

[mydea]

Version

1.50.0

Steps to reproduce

After bumping playwright to 1.50.0, I noticed certain tests failing on Firefox. Digging into stuff, it seems that fetch requests being made with keepalive: true are no longer handled by e.g. page.on('route').

Example:

fetch('https://test-sentry-url.com/', {
  "method": "POST",
  keepalive: true,
});

And my test:

page.route('https://test-sentry-url.com/', route => {
  console.log('Handler met');

  return route.fulfill({
    status: 200,
    contentType: 'application/json',
    body: JSON.stringify({ id: 'test-id' }),
  });
});

Since 1.50.0, the route handler is never triggered (and because of this tests fail). If I remove keepalive: true, it works.

After some debugging, it seems that Firefox actually implemented support for this flag in v133, which has just been bumped to in playwright 1.50.0 (134, specifically). And looking at a regular app in firefox, I can see there that such keepalive requests do not show up in the network tab 🤔 They do seem to be sent though.

Expected behavior

page.route() should continue to handle all requests, even if they have keepalive: true

Actual behavior

page.route() does not match keepalive: true requests.

Additional context

Noticed this here: getsentry/sentry-javascript#15164

Environment

System:
    OS: macOS 14.4.1
    Memory: 1.29 GB / 32.00 GB
  Binaries:
    Node: 18.20.5 - ~/.volta/tools/image/node/18.20.5/bin/node
    Yarn: 1.22.22 - ~/.volta/tools/image/yarn/1.22.22/bin/yarn
    npm: 10.8.2 - ~/.volta/tools/image/node/18.20.5/bin/npm
  Languages:
    Bash: 5.2.26 - /opt/homebrew/bin/bash

[Smrtnyk]

We have the same issue
After playwright update many of our tests failed on firefox

[mxschmitt]

I can repro.