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I tried @yu_tang 's plugin, but it doesn't seem to work properly, maybe because it's not compatible with the latest version of Vanilla. Basically all the test appears as a bit paragraph above the comment box, not as a separate floating block on the side.
After a while I noticed that the Help box has started appearing as it does in this forum, but it is floating on top of the Comment box. The Comment box is not resizing to make room for the Help box.
I think it's a really good feature to have, and will try to play around with the CSS to see if I can get the two boxes to align well.