Mark, on this install of Vanilla (on lussumo), on the dropdowns for category selection... above the "Stickies" category, there is a "---" marker which is not selectable (see the categories dropdown at the top of the panel to the left, or at the top of form when creating a new discussion). Is this hardcoded, or an extension not on the installed extensions list, or some feature I didn't know about?
edit: seems to have an ID of -1, *tries adding a category with ID -1 to his local install*
edit: must be a hack of some sort, http://lussumo.com/community/?CategoryID=-1 that is the --- category on this forum, you can see it puts the Categories dropdown to have the --- selected on the left. I can reproduce on my forum, that by setting a category to have an id of -1, it will not display a page like the link I just provided on this forum... so I imagine this vanilla just has some code to tell the drop down that it should not let it be selectable.
Very strange indeed. The only reason that I am asking, is that one of the extensions I am planning, will involve categories that do not appear on the categories list as a category, but as a sub header... I thought I might be able to manipulate this code to help me.
bshultz is correct. If you go to the category page and block a few categories, and then go back to the discussion page, the ones you blocked will appear below the dashed line. It's a hard-coded thing in the extension, I think.