the best you may possibly have is a so called "snap-to-scale" setting (see menu View > Toolbars > Scale, or View > Scale > ...) where you may select the key note (C, C#, D, etc.) and the scale type or mode (major, minor, pentatonic, etc.) and then new notes are only allowed to be inserted or moved if it falls on that scale.
but no, there's no (vertical) folding in qtractor MIDI clip editor aka piano-roll, sorry.
nope.
the best you may possibly have is a so called "snap-to-scale" setting (see menu View > Toolbars > Scale, or View > Scale > ...) where you may select the key note (C, C#, D, etc.) and the scale type or mode (major, minor, pentatonic, etc.) and then new notes are only allowed to be inserted or moved if it falls on that scale.
but no, there's no (vertical) folding in qtractor MIDI clip editor aka piano-roll, sorry.
hth.
cheers