Panel/spacer border setting.
Right now, there doesn't seem to be a way to add a border (colored or otherwise) to the :spacer within a panel, or to the panel as a whole. This means that any attempt to add e.g. a permanent 1px border to the bottom of the panel as a separator between panel and appspace is currently impossible: There will be gaps in the border in the :spacer sections.
Adding a border property to the :spacer would solve the issue, as would the ability to add a border to the panel as a whole. I hereby request that such a property be added ;)