I have Raspberry pi 2 with Real VNC server 6.1.1. and two Macbook pros, one with Sierra, one with El Capitan, both with Real VNC viewer 6.17.
I have this problem with Sierra only : the bottom of the screen is missing.
I have tried two resolution settings on Raspberry pi : Default 720x480 and DMT 1280x720.
Display on Sierra is still bad. Any idea ?