When i click to customize the main page, there is a button on the right : "Deactivate javascript".

First question: what for is it usefull ?

And secondly, seems i have a bug because when i click on "deactivate javascript", it seems it saves but in fact when i return on the page after saving, the button is still "deactivate javascript" instead of "reactivate". 

You know this bug and how to solve it ?

Thanks.