Basically all rounded rectangles on iOS, including SF Symbols, use squircles (continuously smooth corners) instead of classic circular corners. It would be great if McKinley supported this.
Sketch for example lets you select the “Smooth” corner style to do this: Corners · Sketch
In SwiftUI it’s RoundedCornerStyle.continuous.
Thanks for the suggestion, I’ll have a look at how hard it is to do!
I found this blog post from a quick google:
Would you be wanting the squicle corner rounding just on rectangles, or can you think of a use case for it on arbitrary path corners?
Good question. I think I would only need it for rectangles.
Sketch seems to support it for other shapes as well: “[Smooth corners] work on any angle, including angles greater than 90° (polygons, stars, and complex shapes), and adapt automatically when a shape lacks room for a full smooth curve.”
Maybe better ask a real designer 
Any real designers in the house? 
I’d want to be able to apply the squircle corner to any 90° angle (like say on a right triangle), but can’t think of a case where it would be important for non-90° angle corners. That said, just getting on rectangles would be a great start.
These are in version 1.2.