When tabs with visible background shapes animate across empty regions of the frame, we should probably hide their separators, and fade them back in using the same algorithm we already use for tabs animating to the ends of the strip.

Have to be careful in this case not to hide separators between tabs moving in sync (e.g. drag out the first tab in a strip of several tabs) or when the empty space is not actually showing the frame (e.g. when closing that first tab instead of dragging it out).

Very low priority; it's hard to notice this at our current animation speed.  But it would be nice to make the code more coherent and general about how to deal with the separators on animating tabs.