On a multi-monitor Macintosh setup if the viewport is not smooth ...
SOLUTION 1:
ENABLE "Displays have different spaces"
SOLUTION 2:
IF you want to keep "Displays have different spaces" NOT ENABLED, make sure that you keep the right edge of the window containing the viewport at least 111 pixels away from the right and/or bottom edge of the monitor. This ensures that the automatic 111-pixel drop-shadow that MacOS draws around the window DOES NOT cross over into another display (which compromises the way MacOS draws the viewport).
... or ...
Keep the viewport on the right-most and/or bottom-most monitor.