This will be an amazing addition to Cubase! As someone who spends a large amount of my life working with drum programming this is a GIGANTIC time saver! The continuing collapse of my key editor when smashed into the project window makes that view style more usable. I only see the kit pieces I use and don’t have to switch views to edit. If I need more articulations, I simply press the Fold or Uncollapse button, add the articulation, and fold it again to now see that articulation in the collapsed view.
I just found out about this feature in Logic Pro X when I loaded it up to try out because my workflow in Cubase 10 continues to be sloppy and frustrating. Logic had some real wins over Cubase and collapsing the piano roll to only include the notes I use was one such revelation. PLEASE IMPLEMENT THIS!