For some reason, the first active sidebar when loading the sidebar section can't be configured. The reason why it can't be configured is because, for some reason, the <form> element isn't in the DOM, so the javascript observer can't observe it. The <form>'s children are in the DOM, but the <form> itself isn't. I've been poking at this for a while and I can't figure it out. What I did notice is that if I remove the available plugins box, the problem seems to go away, but I don't see why, and I'm not entirely sure this is the case because of the lack of good tools for this sort of stuff. It's not a case of missing start/end tags, because once I fix some other miscellaneous issues (I'll post a patch about that shortly), the page validates perfectly fine.