Function track_controls 

fn track_controls(
    ui: &mut Ui,
    label: &str,
    sequencer: &mut MidiSequencer,
    layout: &mut PanelLayout,
    audio: &JobSender,
)

One track’s transport: its name, then the buttons its state calls for.

The row names the track so the buttons need no suffix of their own. A sequence that played to its end is idle again, so its row offers the same Play, which returns it to its start before starting it.