WebKit currently hardcodes cell sizes and outsets in order to paint controls on macOS. These values should be programmatically determined using AppKit. `-[NSCell cellSize]` can be used to obtain cell sizes. Obtaining outsets is a little trickier since there is no interface on `NSCell` that exposes these values – `-[NSView alignmentRectInsets]` is one option, but WebKit currently does not create `NSControl`s.
<rdar://problem/104851604>