The text input field for the CAPTCHA when editing (adding a link) is tiny and doesn't use a pretty OOUI field, unlike most of the form. Spotted by @iamjessklein.
(When I've first seen the screenshot, it took me a while to even notice the input…)
The same interface on the signup page (https://en.wikipedia.org/wiki/Special:CreateAccount, logged out) looks much better (I think that has some custom hacks on top of it…)
To reproduce the problem locally, you need these settings:
wfLoadExtensions( array( 'ConfirmEdit', 'ConfirmEdit/FancyCaptcha' ) ); $wgCaptchaClass = 'FancyCaptcha'; $wgCaptchaTriggers['addurl'] = true;
…then edit a page and try to add http://example.net/ to it as an anonymous user.