Currently, C-u RET can be used to schedule a single track for immediate playback after the current track finishes.  This intra-playlist queue should be extended to support multiple queued tracks.

Good keybindings for this are ‘e’ and ‘E’, which should work analogously to how they work in library buffers.  That is, ‘e’ should append to the queue and ‘E’ should enqueue directly to the front of the queue.

For the intra-plalist queue, it does not make sense to enqueue a single track multiple times (at least, it is not feasible for UI purposes).  Therefore, ‘e’ should back-enqueue or move forwards in the queue, and ‘E’ should front-enqueue or move to the front of the queue.

It would also be useful to have dequeuing commands.  For symmetry, ‘d’ should move backwards in the queue or deqeue, and ‘D’ should dequeue immediately.  These keys should also do something useful in library buffers.  Perhaps ‘d’ should move backwards the last occurence of the track in the playlist, while ‘D’ should delete the last occurence.

The key ‘d’ conflicts with ‘bongo-dired-line’, which is the current binding.  I think that needs to move, for the purpose of symmetry.  I’m not sure where, however.

Another thing that would be good for the intra-playlist queue is mouse support.  Romain once suggested clicking in the fringe should manipulate the intra-playlist queue.  I proposed ‘<mouse-1>’ for ‘e’, ‘<C-mouse-1>’ for ‘E’, ‘<mouse-3>’ for ‘d’, and ‘<C-mouse-3>’ for ‘D’.  So clicking next to a track would enqueue it, and right-clicking would dequeue it again.

Finally, it will be necessary to display the intra-playlist queue somehow.  Fringe display would be nice, and would be good for the mouse support.  However, I don’t know whether it is possible to display text in the fringe.  The alternative is to design digit icons, but that would mean we would have an upper limit on the number of queue positions we could display (probably one for each digit, so nine).  This may not actually be a problem in practice, because I don’t expect intra-playlist queues to become very long.  Besides, for long playlists, playlist buffers will be much more convenient anyway, so I almost want this to scale poorly.

So we just need those digit icons and then we can get down to implementing the queue functionality.  Well, or we could do it the other way around.  We’ll see what happens first.