MediaWiki Extension Vector.php prevents sidenav from expanding

SYMPTOMS
On MediaWIki 1.23.0 with the extension Vector.php installed, the side navigation items could not be expanded when closed, they would open and then close again immediately rendering the navigation inoperable.

This problem was only evident when actually using the Vector skin.

I did a fair bit of searching on the web but could not see any reports of similar problems.

SOLUTION
After disabling extensions listed in LocalSettings.php, one-by-one, it turned out to be the entry: require_once("$IP/extensions/Vector/Vector.php");

I'm not entirely sure why it was there in the first place. It's not required to use the Vector skin.

It was an extension that was added to the core on MediaWiki 1.18 but is now obsolete.

I'm pretty certain it did not cause any problems until this mediaWiki was upgraded to 1.23.0

If you notice a similar problem, just comment out the include and your navigation will work properly again.