Manual:$wgVectorUseSimpleSearch/en

Details
Controls search form behavior for Vector skin only:
 * = use one icon search button
 * = use separate "Go" and "Search" buttons. Since MediaWiki 1.23, if JavaScript is enabled in the browser, only the Go button will be shown with $wgVectorUseSimpleSearch = false;. If JavaScript is not enabled, then both buttons will be visible.