I use “Fold MIDI notes” a lot in Ableton’s piano roll MIDI note editor, which gives you the option to fold up and hide unused keys/notes, and find it a really useful editing tool.
I see Cubase 9 has a similar function, but only in Drum editor mode, from what I have researched? (apologies if this already exists- I’m only a week new into learning Cubase!)
Request to please add this function to the standard piano roll MIDI note editor!
I would like to second this, except go a step further than Ableton and be able to define all the possible Scales/Modes on multiple tracks simultaneously.
Folding is a nice feature in Abelton, however, the Drum Map can work in a similar way. Not identical perhaps, but a Drum Map can be set to show only used notes or all the notes used by the kit. If this is an essential feature for you, I’d suggest looking at Drum Maps as an alternative to Folding.