gistfile1.swift

public class SheetAnimator: NSObject, NSViewControllerPresentationAnimator {
    
    private var animationUUID: UUID?

      public func animatePresentation(of viewController: NSViewController, from fromViewController: NSViewController) {
        let containerView = fromViewController.view
        let sheetView = viewController.view
        
        let uuid = UUID()
        animationUUID = uuid
                
        print("\nSheet view origin x was \(sheetView.frame.origin.x)")
        
        containerView.addSubview(sheetView)
        sheetView.frame.origin = CGPoint(x: (containerView.bounds.width - sheetView.bounds.width)/2, y: -sheetView.frame.height)
        
        print("Sheet presentation origin x was \((sheetView.layer!.presentation()?.frame.origin.x))")

        print("Container view width was \(containerView.bounds.width)")
        print("Sheet origin x was set to \(sheetView.frame.origin.x) with width \(sheetView.bounds.width)")
        
        NSAnimationContext.runAnimationGroup({ (context) -> Void in
            context.duration = NSView.defaultAnimationDuration
            sheetView.animator().frame.origin.y = 0
        }, completionHandler: { [weak self] in
            guard uuid == self?.animationUUID else { return }
            NSLayoutConstraint.activate([
                sheetView.bottomAnchor.constraint(equalTo: containerView.bottomAnchor),
                sheetView.centerXAnchor.constraint(equalTo: containerView.centerXAnchor)
            ])
        })
        
    }
    
    public func animateDismissal(of viewController: NSViewController, from fromViewController: NSViewController) {
        let sheetView = viewController.view
        
        let uuid = UUID()
        animationUUID = uuid
        
        sheetView.immediateConstraintsAffectingSelf.deactivate()

        NSAnimationContext.runAnimationGroup({ (context) -> Void in
            context.duration = NSView.defaultAnimationDuration
            sheetView.animator().frame.origin.y = -sheetView.frame.height
        }, completionHandler: { [weak self] in
            guard uuid == self?.animationUUID else { return }
            print("Removed")
            sheetView.removeFromSuperview()
        })
    }
    
}

Output