Here are some of my favorite useful shortcuts and key commands when using the Logic Pro X Piano Roll.
To change or setup these shortcuts, open your Key Command window (Option + K) and search for the shortcuts I mention in the video:
Transpose Event 1 Semitone (Option + Up or Down), / Transpose Event 12 Semitones (Option + Shift + Up or Down)
Trim Note End to Following Notes – Forced Legato (Shift + Backslash)
Trim Note to Remove Overlaps For Adjacent (Backslash)
Repeat Regions / Events (Command + R)
Adjust Note Velocity (Control + Command + Click and Drag)
Collapse Mode (Click “Collapse Mode” button)