PrincipleOfConvexity
The principle of convexity tells you how to clearly distinguish between figure and ground. Figures have convex outlines.
The rounded ends of Apple's scrollbar thumbs clearly distinguish them as figures against the ground of the scrollbar track.
If the scrollbar thumb is rectangular, it can be hard to tell which rectangle is the thumb and which is the track. Example from https://codepen.io/mariaklara/pen/bJjjLY / https://onaircode.com/html-css-custom-scrollbar-examples/
