The horizontal floating subview, when added to the NSScrollView, like with the snippet below, erroneusly adds the top inset twice (see attached screenshot). The floating subviews container view is not located at the begining of the safe content, but instead is located lower. That is not happening for the vertical floating subview:
let scrollView: NSScrollView = ...
scrollView.automaticallyAdjustsContentInsets = true // default
let subview = NSView(frame: CGRect(x: 0, y: 0, width: 20, height: 20))
subview.wantsLayer = true
subview.layer?.backgroundColor = NSColor.red.cgColor
scrollView.addFloatingSubview(subview, for: .horizontal)