The spec says it must be called with a details dict [1]

[1] https://w3c.github.io/mediasession/#callbackdef-mediasessionactionhandler

Given we don't support seeking, only the bare "details" with the action is implemented.

navigator.mediaSession.setActionHandler("nexttrack", (details) => {
    console.log("next track", details);
});

prints next track {action: nexttrack} now instead of next track undefined