Manual:$wgAjaxWatch

Ajax: $wgAjaxWatch
Enable watching/unwatching pages using Ajax.
Introduced in version:1.9.0 (r18598)
Removed in version:1.27.0 (Gerrit change 247729; git #4a68d17b)
Allowed values:(boolean)
Default value:true (1.11-1.26)
false (1.9-1.10)

Details

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Enable watching/unwatching pages using Ajax. Requires $wgUseAjax and $wgEnableAPI to be true too. Causes wfAjaxWatch to be added to $wgAjaxExportList .

In MediaWiki 1.27, $wgAjaxWatch has been removed. The functionality is now always enabled.

Other

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If you want to add a Watch/Unwatch tab (the tab with the star), set $wgVectorUseIconWatch to true.