I considered having this just be a straight guard followed by an
early-return, but if that guard ever actually gets hit, things are going
to be severely broken all over, and un-breaking them at that point will
be very annoying, so just going to cross fingers and hope this can be an
assertion forevermore instead.
This also allows *all* panels to be re-used based on equality, as
originally intended. Beatmap updates should be handled correctly without
a full (flashing) recreation of panels now.