The browser’s user interface contains several pieces of security information. To preserve this information correctly, web page content should not be able to display over the user interface. Certain styling can cause Opera to allow the content to be displayed outside the page, over the address field. This could allow a page to falsify the address field, and create custom dialogs that mimic security dialogs, displaying misleading information about the page’s address and security.
This issue affects Opera for Mac.
Opera Software has released Opera 11.62, where this issue has been fixed.