Manual:$wgUseAjax/en

Enables AJAX support. Required by some extensions and optional features.

Before 1.8.0, enabling Ajax support implicitly activated auto-suggestion for the search bar. Between 1.8 and 1.19 it had to be enabled explicitly: see  (for 1.8 to 1.13) and   (for 1.13 to 1.19). It returned to being implicitly activated when  was removed in 1.20.