All the layers shared one history entry, so the first Back spent it on one layer and the second
left the player with the stats readout (or the countdown) still up. Each layer now holds its own
entry; Back closes the one opened last.
The ⋯ menu, the tracks menu, the tuning panel, the info overlay and the up-next card were a flag
each, so two could stand on top of each other, and Back/Escape were written out three times. They
are one `panel` value now; the stats readout stays a separate toggle beside it. One `back()` peels
the countdown, the panel, then the stats, then leaves — for the arrow, ⌫, Escape and the mouse
Back alike. The mouse Back now also closes the ⋯ menu instead of leaving the player.
peter