If you add a new Username/Password credential, the "OK" button obscures the fields. This happens only after you change the credentials type from something else to Username/Password.
1. Click Credentials link on left
2. Click Global Credentials
3. Click Add Credentials
4. Change the 'Kind' to "Username with Password"
You can work around this by scrolling the screen (perhaps by using pageup/pagedown).
The root issue seems to be that "#bottom-sticker" in hudson-behavior.js needs to attach adjustSticker() on ajax-type events, such as those triggered by changing the dropDownList.