At a very particular content and viewport height, a notebook will shake violently due to the scrollbar toggling constantly. Observable’s frontend code already registers this as a ResizeObserver loop error (encountered on Vega-Lite, Unminified / Fabian Iwand | Observable).
In the below sceencap (taken in incognito mode, after closing the CTA) the document size toggles between 1045x1251.09
and 1060x1253.44
.