I have tracked a couple points and turned on 2D stabilise. It works well. Then I select a rotation track. It works well, however due to the track moving slightly as the point wanders subtly, I have some rotational jitter. That is the footage rotates around the z axis by a few pixels.
Can I smooth the rotation but keep the correction? I guess I could do that in compositor but prefer to use the clip directly in the VSE (Movie clips are available there now!). But it would seem that I cannot smooth the track curves in the Movie Clip editor.
So the solution seems to be, get a better track. The rotation is based on the track sample of a point from frame to frame. You probably wouldn’t notice a standard stabilise in x,y axis, as sub pixel movement gets averaged out (gets a bit blurry). But rotation around z axis can amplify the small errors from a track. Any jitter from poor tracks (you need 2 for rotation of course) will become z axis jitter rolling back and forward by a degree or so (yuck).
My fix was to increase the tracker size, so that it was defining a larger area to more accurately track a feature. The default was way to small (11 units), I made it 3 times bigger. But had to deal with occlusion from people passing infront of the marker sample area.
My “B-plan” was trying to smooth the track path.