If I understand correctly, this concerns MIDI items in Arrangement view, and that their contents are cropped vertically before drawing MIDI item previews.
Quick workaround at the moment of posting:
make a MIDI item that has highest and lowest MIDI notes (velocities can be 1, and item can be muted in Arrangement).
It'll disable vertical cropping of MIDI data before drawing previews, on every track it's present in.
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+1 to option of placing a Mid C line in MIDI items (with blending similar to Photoshop's Overlay etc.)
...but I think having options for both Mid C line and uncropped vertical preview might be ideal. It'd be helpful especially in cases where same musical phrase is doubled by different instruments in different octaves.
Ergo, +1 also to having an option to turn off vertical cropping of MIDI data before drawing a preview of MIDI item.
This would also be well suited for retina / high DPI displays - and planning forward to 5K and 8K displays - because at retina/HiDPI 200% scaling each MIDI pitch could have its own pixel line at same zoom level that shows second row of track controls.