warn users when accidentally changing URLs away when text exists in comment textarea

Currently if input is entered into the Comments textarea similar to this description field, if the user navigates away to another site (accidentally, eg to homepage), the contents of the comment box is lost (it's possible to return and re-click 'comment' to return the textarea).

A customer has suggested a javascript warning popup box would be a nice UI addition (similar to when you attempt to close a tab/page when ANY input has taken place in a textarea.