Allen and I think we'd benefit from a fast (e.g. 100 ms) crossfade for the frame color when switching in/out of single-tab mode.

Things look shippable as-is, so this is P3 polish.

If we crossfade, we need to decide how to deal with the first tab's bounds changing too -- do we snap to the new bounds at the start/end of the crossfade, or do we animate/fade the bounds in some way?  I'd start by snapping at some point and see if it works.