As an anon user in an incognito window
Check beta box
Go back to settings page and you are still in stable
Seen in Chrome on desktop and nexus 5
When logged in this doesn't seem to happen.
- Visit https://en.m.wikipedia.beta.wmflabs.org/wiki/Main_Page (the base domain has a weird redirect) as an anon.
- Visit the mobile settings page by opening the menu and clicking "Settings".
- Check "Beta".
- Click "Save".
- Observe that the optin cookie is set for the .wikipedia.beta.wmflabs.org domain.
- Visit the mobile settings page.
- Observe that "Beta" is checked.
- Switch to the desktop mode of the site by clicking "Desktop" in the footer menu.
- Observe that the stopMobileRedirect cookie is set for the .wikipedia.beta.wmflabs.org domain.
Repeat the above for your local wiki. If you're using MediaWiki-Vagrant, then you can visit both http://127.0.0.1:8080/ and http://dev.wiki.local.wmftest.net:8080, for which the cookie domains should be 127.0.0.1 and .wiki.local.wmftest.net respectively.