Thanks for your quick reply and suggestions. Switching on Legacy Compositing does not make any difference Im afraid. Turing off the graphics processor setting does not give a live update, so changes the whole behaviour.
Im using a Retina iMac 32GB running Mojave.
I've attached a screengrab .mov to show what happens. I use normal (proportional scale) first, then do it again holding shift, and you can see the jitter, but can't post it here.