This feature appears to be completely nuked in the latest desktop version (I'm on OSX).
Typically, my usage pattern is to queue up a bunch of music and forget about it. When I double click a song, it will stuff the Play Queue with a bunch of stuff I don't want to listen to. Before, I had the option to swap out the Play Queue with songs from another playlist using the "Choose as current playlist" feature (so when the stuff I queue up is done playing, it will switch over to the playlist). I also use this feature to indicate that I don't want any songs to play after the things I've queued up (by choosing an empty playlist as the "current playlist").
Without this functionality, the Play Queue is stuck there. Yes, I can doubleclick a new song somewhere else, but that will interrupt the song that is currently playing. In general we need a better way to manage the music that autoplays after the queued songs.
Updated on 2019-12-04
Considering this idea is rather old and we're not looking at implementing it, we'll close this now.
If something should change we'll be back with an update.