Screenshot showing the problem
When simple search (from the Vector extension) is enabled, the suggestion box is displayed twice (in different locations on the screen, apparently partially depending on the size of the window). See attached screenshot.
I have reproduced this issue on the current SVN version (both MediaWiki itself and the Vector extension were up-to-date). It can also be seen on the commons prototype page.
This is using Chromium 6.0.472.63~r59945-2 as shipped with current Debian Testing.