Well, you know what I will say: title says it all !

I would like that in the collection panel, there is the possibility to choose "Disc number" as the third level of sorting. So if there is a disc number tag in one of the tracks of the album, then a third level appear with "Disc 1". If there is no disc number in one track while some other tracks of the same disc has one, then it is put under "Unkown". If there is no disc number in none of the tracks of an album, then there is no third level at all (not a "unkown" disc number for all tracks).

If a disc number is found in one track of that album

Artist
  Album
      Unkown
        Track title
      Disc 1
        Track title
      Disc 2
        Track title


If no disc number is found in all tracks of that album

Artist
  Album
      Track title

Essential, yep!

I've been searching for this for a while this morning... has this been addressed yet?
I'm not a fan of having 4 discs in a set listed individually..it would make a lot more sense to have the Disc number as the 3rd level of sorting

Please subscribe to this bug: https://bugs.kde.org/show_bug.cgi?id=230362. Currently the disc number is not even part of the possible filters in the list.